This file is indexed.

/usr/share/perl/5.14.2/pod/perlop.pod is in perl-doc 5.14.2-6ubuntu2.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

   1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
=head1 NAME
X<operator>

perlop - Perl operators and precedence

=head1 DESCRIPTION

=head2 Operator Precedence and Associativity
X<operator, precedence> X<precedence> X<associativity>

Operator precedence and associativity work in Perl more or less like
they do in mathematics.

I<Operator precedence> means some operators are evaluated before
others.  For example, in C<2 + 4 * 5>, the multiplication has higher
precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
22> and not C<6 * 5 == 30>.

I<Operator associativity> defines what happens if a sequence of the
same operators is used one after another: whether the evaluator will
evaluate the left operations first or the right.  For example, in C<8
- 4 - 2>, subtraction is left associative so Perl evaluates the
expression left to right.  C<8 - 4> is evaluated first making the
expression C<4 - 2 == 2> and not C<8 - 2 == 6>.

Perl operators have the following associativity and precedence,
listed from highest precedence to lowest.  Operators borrowed from
C keep the same precedence relationship with each other, even where
C's precedence is slightly screwy.  (This makes learning Perl easier
for C folks.)  With very few exceptions, these all operate on scalar
values only, not array values.

    left	terms and list operators (leftward)
    left	->
    nonassoc	++ --
    right	**
    right	! ~ \ and unary + and -
    left	=~ !~
    left	* / % x
    left	+ - .
    left	<< >>
    nonassoc	named unary operators
    nonassoc	< > <= >= lt gt le ge
    nonassoc	== != <=> eq ne cmp ~~
    left	&
    left	| ^
    left	&&
    left	|| //
    nonassoc	..  ...
    right	?:
    right	= += -= *= etc.
    left	, =>
    nonassoc	list operators (rightward)
    right	not
    left	and
    left	or xor

In the following sections, these operators are covered in precedence order.

Many operators can be overloaded for objects.  See L<overload>.

=head2 Terms and List Operators (Leftward)
X<list operator> X<operator, list> X<term>

A TERM has the highest precedence in Perl.  They include variables,
quote and quote-like operators, any expression in parentheses,
and any function whose arguments are parenthesized.  Actually, there
aren't really functions in this sense, just list operators and unary
operators behaving as functions because you put parentheses around
the arguments.  These are all documented in L<perlfunc>.

If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
just like a normal function call.

In the absence of parentheses, the precedence of list operators such as
C<print>, C<sort>, or C<chmod> is either very high or very low depending on
whether you are looking at the left side or the right side of the operator.
For example, in

    @ary = (1, 3, sort 4, 2);
    print @ary;		# prints 1324

the commas on the right of the sort are evaluated before the sort,
but the commas on the left are evaluated after.  In other words,
list operators tend to gobble up all arguments that follow, and
then act like a simple TERM with regard to the preceding expression.
Be careful with parentheses:

    # These evaluate exit before doing the print:
    print($foo, exit);	# Obviously not what you want.
    print $foo, exit;	# Nor is this.

    # These do the print before evaluating exit:
    (print $foo), exit;	# This is what you want.
    print($foo), exit;	# Or this.
    print ($foo), exit;	# Or even this.

Also note that

    print ($foo & 255) + 1, "\n";

probably doesn't do what you expect at first glance.  The parentheses
enclose the argument list for C<print> which is evaluated (printing
the result of C<$foo & 255>).  Then one is added to the return value
of C<print> (usually 1).  The result is something like this:

    1 + 1, "\n";    # Obviously not what you meant.

To do what you meant properly, you must write:

    print(($foo & 255) + 1, "\n");

See L<Named Unary Operators> for more discussion of this.

Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
well as subroutine and method calls, and the anonymous
constructors C<[]> and C<{}>.

See also L<Quote and Quote-like Operators> toward the end of this section,
as well as L</"I/O Operators">.

=head2 The Arrow Operator
X<arrow> X<dereference> X<< -> >>

"C<< -> >>" is an infix dereference operator, just as it is in C
and C++.  If the right side is either a C<[...]>, C<{...}>, or a
C<(...)> subscript, then the left side must be either a hard or
symbolic reference to an array, a hash, or a subroutine respectively.
(Or technically speaking, a location capable of holding a hard
reference, if it's an array or hash reference being used for
assignment.)  See L<perlreftut> and L<perlref>.

Otherwise, the right side is a method name or a simple scalar
variable containing either the method name or a subroutine reference,
and the left side must be either an object (a blessed reference)
or a class name (that is, a package name).  See L<perlobj>.

=head2 Auto-increment and Auto-decrement
X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->

"++" and "--" work as in C.  That is, if placed before a variable,
they increment or decrement the variable by one before returning the
value, and if placed after, increment or decrement after returning the
value.

    $i = 0;  $j = 0;
    print $i++;  # prints 0
    print ++$j;  # prints 1

Note that just as in C, Perl doesn't define B<when> the variable is
incremented or decremented. You just know it will be done sometime
before or after the value is returned. This also means that modifying
a variable twice in the same statement will lead to undefined behavior.
Avoid statements like:

    $i = $i ++;
    print ++ $i + $i ++;

Perl will not guarantee what the result of the above statements is.

The auto-increment operator has a little extra builtin magic to it.  If
you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment.  If, however, the
variable has been used in only string contexts since it was set, and
has a value that is not the empty string and matches the pattern
C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
character within its range, with carry:

    print ++($foo = "99");	# prints "100"
    print ++($foo = "a0");	# prints "a1"
    print ++($foo = "Az");	# prints "Ba"
    print ++($foo = "zz");	# prints "aaa"

C<undef> is always treated as numeric, and in particular is changed
to C<0> before incrementing (so that a post-increment of an undef value
will return C<0> rather than C<undef>).

The auto-decrement operator is not magical.

=head2 Exponentiation
X<**> X<exponentiation> X<power>

Binary "**" is the exponentiation operator.  It binds even more
tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
implemented using C's pow(3) function, which actually works on doubles
internally.)

=head2 Symbolic Unary Operators
X<unary operator> X<operator, unary>

Unary "!" performs logical negation, i.e., "not".  See also C<not> for a lower
precedence version of this.
X<!>

Unary "-" performs arithmetic negation if the operand is numeric,
including any string that looks like a number.  If the operand is
an identifier, a string consisting of a minus sign concatenated
with the identifier is returned.  Otherwise, if the string starts
with a plus or minus, a string starting with the opposite sign is
returned.  One effect of these rules is that -bareword is equivalent
to the string "-bareword".  If, however, the string begins with a
non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
the string to a numeric and the arithmetic negation is performed. If the
string cannot be cleanly converted to a numeric, Perl will give the warning
B<Argument "the string" isn't numeric in negation (-) at ...>.
X<-> X<negation, arithmetic>

Unary "~" performs bitwise negation, i.e., 1's complement.  For
example, C<0666 & ~027> is 0640.  (See also L<Integer Arithmetic> and
L<Bitwise String Operators>.)  Note that the width of the result is
platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
bits wide on a 64-bit platform, so if you are expecting a certain bit
width, remember to use the "&" operator to mask off the excess bits.
X<~> X<negation, binary>

When complementing strings, if all characters have ordinal values under
256, then their complements will, also.  But if they do not, all
characters will be in either 32- or 64-bit complements, depending on your
architecture.  So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.

Unary "+" has no effect whatsoever, even on strings.  It is useful
syntactically for separating a function name from a parenthesized expression
that would otherwise be interpreted as the complete list of function
arguments.  (See examples above under L<Terms and List Operators (Leftward)>.)
X<+>

Unary "\" creates a reference to whatever follows it.  See L<perlreftut>
and L<perlref>.  Do not confuse this behavior with the behavior of
backslash within a string, although both forms do convey the notion
of protecting the next thing from interpolation.
X<\> X<reference> X<backslash>

=head2 Binding Operators
X<binding> X<operator, binding> X<=~> X<!~>

Binary "=~" binds a scalar expression to a pattern match.  Certain operations
search or modify the string $_ by default.  This operator makes that kind
of operation work on some other string.  The right argument is a search
pattern, substitution, or transliteration.  The left argument is what is
supposed to be searched, substituted, or transliterated instead of the default
$_.  When used in scalar context, the return value generally indicates the
success of the operation.  The exceptions are substitution (s///)
and transliteration (y///) with the C</r> (non-destructive) option,
which cause the B<r>eturn value to be the result of the substitution.
Behavior in list context depends on the particular operator.
See L</"Regexp Quote-Like Operators"> for details and L<perlretut> for
examples using these operators.

If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern at run
time. Note that this means that its contents will be interpolated twice, so

  '\\' =~ q'\\';

is not ok, as the regex engine will end up trying to compile the
pattern C<\>, which it will consider a syntax error.

Binary "!~" is just like "=~" except the return value is negated in
the logical sense.

Binary "!~" with a non-destructive substitution (s///r) or transliteration
(y///r) is a syntax error.

=head2 Multiplicative Operators
X<operator, multiplicative>

Binary "*" multiplies two numbers.
X<*>

Binary "/" divides two numbers.
X</> X<slash>

Binary "%" is the modulo operator, which computes the division
remainder of its first argument with respect to its second argument.
Given integer
operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
C<$a> minus the largest multiple of C<$b> less than or equal to
C<$a>.  If C<$b> is negative, then C<$a % $b> is C<$a> minus the
smallest multiple of C<$b> that is not less than C<$a> (i.e. the
result will be less than or equal to zero).  If the operands
C<$a> and C<$b> are floating point values and the absolute value of
C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
the integer portion of C<$a> and C<$b> will be used in the operation
(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
If the absolute value of the right operand (C<abs($b)>) is greater than
or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
integer that makes C<$r> have the same sign as the right operand
C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
and the absolute value less than that of C<$b>.
Note that when C<use integer> is in scope, "%" gives you direct access
to the modulo operator as implemented by your C compiler.  This
operator is not as well defined for negative operands, but it will
execute faster.
X<%> X<remainder> X<modulo> X<mod>

Binary "x" is the repetition operator.  In scalar context or if the left
operand is not enclosed in parentheses, it returns a string consisting
of the left operand repeated the number of times specified by the right
operand.  In list context, if the left operand is enclosed in
parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
If the right operand is zero or negative, it returns an empty string
or an empty list, depending on the context.
X<x>

    print '-' x 80;		# print row of dashes

    print "\t" x ($tab/8), ' ' x ($tab%8);	# tab over

    @ones = (1) x 80;		# a list of 80 1's
    @ones = (5) x @ones;	# set all elements to 5


=head2 Additive Operators
X<operator, additive>

Binary "+" returns the sum of two numbers.
X<+>

Binary "-" returns the difference of two numbers.
X<->

Binary "." concatenates two strings.
X<string, concatenation> X<concatenation>
X<cat> X<concat> X<concatenate> X<.>

=head2 Shift Operators
X<shift operator> X<operator, shift> X<<< << >>>
X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
X<shl> X<shr> X<shift, right> X<shift, left>

Binary "<<" returns the value of its left argument shifted left by the
number of bits specified by the right argument.  Arguments should be
integers.  (See also L<Integer Arithmetic>.)

Binary ">>" returns the value of its left argument shifted right by
the number of bits specified by the right argument.  Arguments should
be integers.  (See also L<Integer Arithmetic>.)

Note that both "<<" and ">>" in Perl are implemented directly using
"<<" and ">>" in C.  If C<use integer> (see L<Integer Arithmetic>) is
in force then signed C integers are used, else unsigned C integers are
used.  Either way, the implementation isn't going to generate results
larger than the size of the integer type Perl was built with (32 bits
or 64 bits).

The result of overflowing the range of the integers is undefined
because it is undefined also in C.  In other words, using 32-bit
integers, C<< 1 << 32 >> is undefined.  Shifting by a negative number
of bits is also undefined.

=head2 Named Unary Operators
X<operator, named unary>

The various named unary operators are treated as functions with one
argument, with optional parentheses.

If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
just like a normal function call.  For example,
because named unary operators are higher precedence than ||:

    chdir $foo    || die;	# (chdir $foo) || die
    chdir($foo)   || die;	# (chdir $foo) || die
    chdir ($foo)  || die;	# (chdir $foo) || die
    chdir +($foo) || die;	# (chdir $foo) || die

but, because * is higher precedence than named operators:

    chdir $foo * 20;	# chdir ($foo * 20)
    chdir($foo) * 20;	# (chdir $foo) * 20
    chdir ($foo) * 20;	# (chdir $foo) * 20
    chdir +($foo) * 20;	# chdir ($foo * 20)

    rand 10 * 20;	# rand (10 * 20)
    rand(10) * 20;	# (rand 10) * 20
    rand (10) * 20;	# (rand 10) * 20
    rand +(10) * 20;	# rand (10 * 20)

Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
treated like named unary operators, but they don't follow this functional
parenthesis rule.  That means, for example, that C<-f($file).".bak"> is
equivalent to C<-f "$file.bak">.
X<-X> X<filetest> X<operator, filetest>

See also L<"Terms and List Operators (Leftward)">.

=head2 Relational Operators
X<relational operator> X<operator, relational>

Binary "<" returns true if the left argument is numerically less than
the right argument.
X<< < >>

Binary ">" returns true if the left argument is numerically greater
than the right argument.
X<< > >>

Binary "<=" returns true if the left argument is numerically less than
or equal to the right argument.
X<< <= >>

Binary ">=" returns true if the left argument is numerically greater
than or equal to the right argument.
X<< >= >>

Binary "lt" returns true if the left argument is stringwise less than
the right argument.
X<< lt >>

Binary "gt" returns true if the left argument is stringwise greater
than the right argument.
X<< gt >>

Binary "le" returns true if the left argument is stringwise less than
or equal to the right argument.
X<< le >>

Binary "ge" returns true if the left argument is stringwise greater
than or equal to the right argument.
X<< ge >>

=head2 Equality Operators
X<equality> X<equal> X<equals> X<operator, equality>

Binary "==" returns true if the left argument is numerically equal to
the right argument.
X<==>

Binary "!=" returns true if the left argument is numerically not equal
to the right argument.
X<!=>

Binary "<=>" returns -1, 0, or 1 depending on whether the left
argument is numerically less than, equal to, or greater than the right
argument.  If your platform supports NaNs (not-a-numbers) as numeric
values, using them with "<=>" returns undef.  NaN is not "<", "==", ">",
"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
returns true, as does NaN != anything else. If your platform doesn't
support NaNs then NaN is just a string with numeric value 0.
X<< <=> >> X<spaceship>

    perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
    perl -le '$a = "NaN"; print "NaN support here" if $a != $a'

Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
X<eq>

Binary "ne" returns true if the left argument is stringwise not equal
to the right argument.
X<ne>

Binary "cmp" returns -1, 0, or 1 depending on whether the left
argument is stringwise less than, equal to, or greater than the right
argument.
X<cmp>

Binary "~~" does a smart match between its arguments. Smart matching
is described in L<perlsyn/"Smart matching in detail">.
X<~~>

"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
by the current locale if C<use locale> is in effect.  See L<perllocale>.

=head2 Bitwise And
X<operator, bitwise, and> X<bitwise and> X<&>

Binary "&" returns its operands ANDed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)

Note that "&" has lower priority than relational operators, so for example
the brackets are essential in a test like

	print "Even\n" if ($x & 1) == 0;

=head2 Bitwise Or and Exclusive Or
X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
X<bitwise xor> X<^>

Binary "|" returns its operands ORed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)

Binary "^" returns its operands XORed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)

Note that "|" and "^" have lower priority than relational operators, so
for example the brackets are essential in a test like

	print "false\n" if (8 | 2) != 10;

=head2 C-style Logical And
X<&&> X<logical and> X<operator, logical, and>

Binary "&&" performs a short-circuit logical AND operation.  That is,
if the left operand is false, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it
is evaluated.

=head2 C-style Logical Or
X<||> X<operator, logical, or>

Binary "||" performs a short-circuit logical OR operation.  That is,
if the left operand is true, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it
is evaluated.

=head2 C-style Logical Defined-Or
X<//> X<operator, logical, defined-or>

Although it has no direct equivalent in C, Perl's C<//> operator is related
to its C-style or.  In fact, it's exactly the same as C<||>, except that it
tests the left hand side's definedness instead of its truth.  Thus, C<$a // $b>
is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
rather than the value of C<defined($a)>) and yields the same result as
C<defined($a) ? $a : $b> (except that the ternary-operator form can be
used as a lvalue, while C<$a // $b> cannot).  This is very useful for
providing default values for variables.  If you actually want to test if
at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>.

The C<||>, C<//> and C<&&> operators return the last value evaluated
(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
portable way to find out the home directory might be:

    $home =  $ENV{HOME}
	  // $ENV{LOGDIR}
	  // (getpwuid($<))[7]
	  // die "You're homeless!\n";

In particular, this means that you shouldn't use this
for selecting between two aggregates for assignment:

    @a = @b || @c;		# this is wrong
    @a = scalar(@b) || @c;	# really meant this
    @a = @b ? @b : @c;		# this works fine, though

As more readable alternatives to C<&&> and C<||> when used for
control flow, Perl provides the C<and> and C<or> operators (see below).
The short-circuit behavior is identical.  The precedence of "and"
and "or" is much lower, however, so that you can safely use them after a
list operator without the need for parentheses:

    unlink "alpha", "beta", "gamma"
	    or gripe(), next LINE;

With the C-style operators that would have been written like this:

    unlink("alpha", "beta", "gamma")
	    || (gripe(), next LINE);

Using "or" for assignment is unlikely to do what you want; see below.

=head2 Range Operators
X<operator, range> X<range> X<..> X<...>

Binary ".." is the range operator, which is really two different
operators depending on the context.  In list context, it returns a
list of values counting (up by ones) from the left value to the right
value.  If the left value is greater than the right value then it
returns the empty list.  The range operator is useful for writing
C<foreach (1..10)> loops and for doing slice operations on arrays. In
the current implementation, no temporary array is created when the
range operator is used as the expression in C<foreach> loops, but older
versions of Perl might burn a lot of memory when you write something
like this:

    for (1 .. 1_000_000) {
	# code
    }

The range operator also works on strings, using the magical
auto-increment, see below.

In scalar context, ".." returns a boolean value.  The operator is
bistable, like a flip-flop, and emulates the line-range (comma)
operator of B<sed>, B<awk>, and various editors. Each ".." operator
maintains its own boolean state, even across calls to a subroutine
that contains it. It is false as long as its left operand is false.
Once the left operand is true, the range operator stays true until the
right operand is true, I<AFTER> which the range operator becomes false
again.  It doesn't become false till the next time the range operator
is evaluated.  It can test the right operand and become false on the
same evaluation it became true (as in B<awk>), but it still returns
true once. If you don't want it to test the right operand until the
next evaluation, as in B<sed>, just use three dots ("...") instead of
two.  In all other regards, "..." behaves just like ".." does.

The right operand is not evaluated while the operator is in the
"false" state, and the left operand is not evaluated while the
operator is in the "true" state.  The precedence is a little lower
than || and &&.  The value returned is either the empty string for
false, or a sequence number (beginning with 1) for true.  The sequence
number is reset for each range encountered.  The final sequence number
in a range has the string "E0" appended to it, which doesn't affect
its numeric value, but gives you something to search for if you want
to exclude the endpoint.  You can exclude the beginning point by
waiting for the sequence number to be greater than 1.

If either operand of scalar ".." is a constant expression,
that operand is considered true if it is equal (C<==>) to the current
input line number (the C<$.> variable).

To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
but that is only an issue if you use a floating point expression; when
implicitly using C<$.> as described in the previous paragraph, the
comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
is set to a floating point value and you are not reading from a file.
Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
you want in scalar context because each of the operands are evaluated
using their integer representation.

Examples:

As a scalar operator:

    if (101 .. 200) { print; } # print 2nd hundred lines, short for
                               #  if ($. == 101 .. $. == 200) { print; }

    next LINE if (1 .. /^$/);  # skip header lines, short for
                               #   next LINE if ($. == 1 .. /^$/);
                               # (typically in a loop labeled LINE)

    s/^/> / if (/^$/ .. eof());  # quote body

    # parse mail messages
    while (<>) {
        $in_header =   1  .. /^$/;
        $in_body   = /^$/ .. eof;
        if ($in_header) {
            # do something
        } else { # in body
            # do something else
        }
    } continue {
        close ARGV if eof;             # reset $. each file
    }

Here's a simple example to illustrate the difference between
the two range operators:

    @lines = ("   - Foo",
              "01 - Bar",
              "1  - Baz",
              "   - Quux");

    foreach (@lines) {
        if (/0/ .. /1/) {
            print "$_\n";
        }
    }

This program will print only the line containing "Bar". If
the range operator is changed to C<...>, it will also print the
"Baz" line.

And now some examples as a list operator:

    for (101 .. 200) { print; }	# print $_ 100 times
    @foo = @foo[0 .. $#foo];	# an expensive no-op
    @foo = @foo[$#foo-4 .. $#foo];	# slice last 5 items

The range operator (in list context) makes use of the magical
auto-increment algorithm if the operands are strings.  You
can say

    @alphabet = ("A" .. "Z");

to get all normal letters of the English alphabet, or

    $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];

to get a hexadecimal digit, or

    @z2 = ("01" .. "31");  print $z2[$mday];

to get dates with leading zeros.

If the final value specified is not in the sequence that the magical
increment would produce, the sequence goes until the next value would
be longer than the final value specified.

If the initial value specified isn't part of a magical increment
sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
only the initial value will be returned.  So the following will only
return an alpha:

    use charnames "greek";
    my @greek_small =  ("\N{alpha}" .. "\N{omega}");

To get the 25 traditional lowercase Greek letters, including both sigmas,
you could use this instead:

    use charnames "greek";
    my @greek_small =  map { chr }
		       ord "\N{alpha}" .. ord "\N{omega}";

However, because there are I<many> other lowercase Greek characters than
just those, to match lowercase Greek characters in a regular expression,
you would use the pattern C</(?:(?=\p{Greek})\p{Lower})+/>.

Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
return two elements in list context.

    @list = (2.18 .. 3.14); # same as @list = (2 .. 3);

=head2 Conditional Operator
X<operator, conditional> X<operator, ternary> X<ternary> X<?:>

Ternary "?:" is the conditional operator, just as in C.  It works much
like an if-then-else.  If the argument before the ? is true, the
argument before the : is returned, otherwise the argument after the :
is returned.  For example:

    printf "I have %d dog%s.\n", $n,
	    ($n == 1) ? "" : "s";

Scalar or list context propagates downward into the 2nd
or 3rd argument, whichever is selected.

    $a = $ok ? $b : $c;  # get a scalar
    @a = $ok ? @b : @c;  # get an array
    $a = $ok ? @b : @c;  # oops, that's just a count!

The operator may be assigned to if both the 2nd and 3rd arguments are
legal lvalues (meaning that you can assign to them):

    ($a_or_b ? $a : $b) = $c;

Because this operator produces an assignable result, using assignments
without parentheses will get you in trouble.  For example, this:

    $a % 2 ? $a += 10 : $a += 2

Really means this:

    (($a % 2) ? ($a += 10) : $a) += 2

Rather than this:

    ($a % 2) ? ($a += 10) : ($a += 2)

That should probably be written more simply as:

    $a += ($a % 2) ? 10 : 2;

=head2 Assignment Operators
X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
X<%=> X<^=> X<x=>

"=" is the ordinary assignment operator.

Assignment operators work as in C.  That is,

    $a += 2;

is equivalent to

    $a = $a + 2;

although without duplicating any side effects that dereferencing the lvalue
might trigger, such as from tie().  Other assignment operators work similarly.
The following are recognized:

    **=    +=    *=    &=    <<=    &&=
           -=    /=    |=    >>=    ||=
           .=    %=    ^=           //=
                 x=

Although these are grouped by family, they all have the precedence
of assignment.

Unlike in C, the scalar assignment operator produces a valid lvalue.
Modifying an assignment is equivalent to doing the assignment and
then modifying the variable that was assigned to.  This is useful
for modifying a copy of something, like this:

    ($tmp = $global) =~ tr [0-9] [a-j];

Likewise,

    ($a += 2) *= 3;

is equivalent to

    $a += 2;
    $a *= 3;

Similarly, a list assignment in list context produces the list of
lvalues assigned to, and a list assignment in scalar context returns
the number of elements produced by the expression on the right hand
side of the assignment.

=head2 The Triple-Dot Operator
X<...> X<... operator> X<yada-yada operator> X<whatever operator>
X<triple-dot operator>

The triple-dot operator, C<...>, sometimes called the "whatever operator", the
"yada-yada operator", or the "I<et cetera>" operator, is a placeholder for
code.  Perl parses it without error, but when you try to execute a whatever,
it throws an exception with the text C<Unimplemented>:

    sub unimplemented { ... }

    eval { unimplemented() };
    if ($@ eq "Unimplemented" ) {
	say "Oh look, an exception--whatever.";
    }

You can only use the triple-dot operator to stand in for a complete statement.
These examples of the triple-dot work:

    { ... }

    sub foo { ... }

    ...;

    eval { ... };

    sub foo {
	my ($self) = shift;
	...;
    }

    do {
	my $variable;
	...;
	say "Hurrah!";
    } while $cheering;

The yada-yada--or whatever--cannot stand in for an expression that is
part of a larger statement since the C<...> is also the three-dot version
of the binary range operator (see L<Range Operators>).  These examples of
the whatever operator are still syntax errors:

    print ...;

    open(PASSWD, ">", "/dev/passwd") or ...;

    if ($condition && ...) { say "Hello" }

There are some cases where Perl can't immediately tell the difference
between an expression and a statement. For instance, the syntax for a
block and an anonymous hash reference constructor look the same unless
there's something in the braces that give Perl a hint. The whatever
is a syntax error if Perl doesn't guess that the C<{ ... }> is a
block. In that case, it doesn't think the C<...> is the whatever
because it's expecting an expression instead of a statement:

    my @transformed = map { ... } @input;  # syntax error

You can use a C<;> inside your block to denote that the C<{ ... }> is
a block and not a hash reference constructor. Now the whatever works:

    my @transformed = map {; ... } @input; # ; disambiguates

    my @transformed = map { ...; } @input; # ; disambiguates

=head2 Comma Operator
X<comma> X<operator, comma> X<,>

Binary "," is the comma operator.  In scalar context it evaluates
its left argument, throws that value away, then evaluates its right
argument and returns that value.  This is just like C's comma operator.

In list context, it's just the list argument separator, and inserts
both its arguments into the list.  These arguments are also evaluated
from left to right.

The C<< => >> operator is a synonym for the comma except that it causes
its left operand to be interpreted as a string if it begins with a letter
or underscore and is composed only of letters, digits and underscores.
This includes operands that might otherwise be interpreted as operators,
constants, single number v-strings or function calls. If in doubt about
this behavior, the left operand can be quoted explicitly.

Otherwise, the C<< => >> operator behaves exactly as the comma operator
or list argument separator, according to context.

For example:

    use constant FOO => "something";

    my %h = ( FOO => 23 );

is equivalent to:

    my %h = ("FOO", 23);

It is I<NOT>:

    my %h = ("something", 23);

The C<< => >> operator is helpful in documenting the correspondence
between keys and values in hashes, and other paired elements in lists.

        %hash = ( $key => $value );
        login( $username => $password );

=head2 List Operators (Rightward)
X<operator, list, rightward> X<list operator>

On the right side of a list operator, the comma has very low precedence,
such that it controls all comma-separated expressions found there.
The only operators with lower precedence are the logical operators
"and", "or", and "not", which may be used to evaluate calls to list
operators without the need for extra parentheses:

    open HANDLE, "< $file"
	or die "Can't open $file: $!\n";

See also discussion of list operators in L<Terms and List Operators (Leftward)>.

=head2 Logical Not
X<operator, logical, not> X<not>

Unary "not" returns the logical negation of the expression to its right.
It's the equivalent of "!" except for the very low precedence.

=head2 Logical And
X<operator, logical, and> X<and>

Binary "and" returns the logical conjunction of the two surrounding
expressions.  It's equivalent to C<&&> except for the very low
precedence.  This means that it short-circuits: the right
expression is evaluated only if the left expression is true.

=head2 Logical or, Defined or, and Exclusive Or
X<operator, logical, or> X<operator, logical, xor>
X<operator, logical, defined or> X<operator, logical, exclusive or>
X<or> X<xor>

Binary "or" returns the logical disjunction of the two surrounding
expressions.  It's equivalent to C<||> except for the very low precedence.
This makes it useful for control flow:

    print FH $data		or die "Can't write to FH: $!";

This means that it short-circuits: the right expression is evaluated
only if the left expression is false.  Due to its precedence, you must
be careful to avoid using it as replacement for the C<||> operator.
It usually works out better for flow control than in assignments:

    $a = $b or $c;		# bug: this is wrong
    ($a = $b) or $c;		# really means this
    $a = $b || $c;		# better written this way

However, when it's a list-context assignment and you're trying to use
C<||> for control flow, you probably need "or" so that the assignment
takes higher precedence.

    @info = stat($file) || die;     # oops, scalar sense of stat!
    @info = stat($file) or die;     # better, now @info gets its due

Then again, you could always use parentheses.

Binary "xor" returns the exclusive-OR of the two surrounding expressions.
It cannot short-circuit (of course).

=head2 C Operators Missing From Perl
X<operator, missing from perl> X<&> X<*>
X<typecasting> X<(TYPE)>

Here is what C has that Perl doesn't:

=over 8

=item unary &

Address-of operator.  (But see the "\" operator for taking a reference.)

=item unary *

Dereference-address operator. (Perl's prefix dereferencing
operators are typed: $, @, %, and &.)

=item (TYPE)

Type-casting operator.

=back

=head2 Quote and Quote-like Operators
X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
X<escape sequence> X<escape>

While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities.  Perl provides customary quote characters
for these behaviors, but also provides a way for you to choose your
quote character for any of them.  In the following table, a C<{}> represents
any pair of delimiters you choose.

    Customary  Generic        Meaning	     Interpolates
	''	 q{}	      Literal		  no
	""	qq{}	      Literal		  yes
	``	qx{}	      Command		  yes*
		qw{}	     Word list		  no
	//	 m{}	   Pattern match	  yes*
		qr{}	      Pattern		  yes*
		 s{}{}	    Substitution	  yes*
		tr{}{}	  Transliteration	  no (but see below)
		 y{}{}	  Transliteration	  no (but see below)
        <<EOF                 here-doc            yes*

	* unless the delimiter is ''.

Non-bracketing delimiters use the same character fore and aft, but the four
sorts of ASCII brackets (round, angle, square, curly) all nest, which means
that

    q{foo{bar}baz}

is the same as

    'foo{bar}baz'

Note, however, that this does not always work for quoting Perl code:

    $s = q{ if($a eq "}") ... }; # WRONG

is a syntax error. The C<Text::Balanced> module (standard as of v5.8,
and from CPAN before then) is able to do this properly.

There can be whitespace between the operator and the quoting
characters, except when C<#> is being used as the quoting character.
C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
operator C<q> followed by a comment.  Its argument will be taken
from the next line.  This allows you to write:

    s {foo}  # Replace foo
      {bar}  # with bar.

The following escape sequences are available in constructs that interpolate,
and in transliterations:
X<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
X<\o{}>

    Sequence     Note  Description
    \t                  tab               (HT, TAB)
    \n                  newline           (NL)
    \r                  return            (CR)
    \f                  form feed         (FF)
    \b                  backspace         (BS)
    \a                  alarm (bell)      (BEL)
    \e                  escape            (ESC)
    \x{263A}     [1,8]  hex char          (example: SMILEY)
    \x1b         [2,8]  restricted range hex char (example: ESC)
    \N{name}     [3]    named Unicode character or character sequence
    \N{U+263D}   [4,8]  Unicode character (example: FIRST QUARTER MOON)
    \c[          [5]    control char      (example: chr(27))
    \o{23072}    [6,8]  octal char        (example: SMILEY)
    \033         [7,8]  restricted range octal char  (example: ESC)

=over 4

=item [1]

The result is the character specified by the hexadecimal number between
the braces.  See L</[8]> below for details on which character.

Only hexadecimal digits are valid between the braces. If an invalid
character is encountered, a warning will be issued and the invalid
character and all subsequent characters (valid or invalid) within the
braces will be discarded.

If there are no valid digits between the braces, the generated character is
the NULL character (C<\x{00}>).  However, an explicit empty brace (C<\x{}>)
will not cause a warning (currently).

=item [2]

The result is the character specified by the hexadecimal number in the range
0x00 to 0xFF.  See L</[8]> below for details on which character.

Only hexadecimal digits are valid following C<\x>.  When C<\x> is followed
by fewer than two valid digits, any valid digits will be zero-padded.  This
means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be
interpreted as C<\x00>.  Except at the end of a string, having fewer than
two valid digits will result in a warning.  Note that although the warning
says the illegal character is ignored, it is only ignored as part of the
escape and will still be used as the subsequent character in the string.
For example:

  Original    Result    Warns?
  "\x7"       "\x07"    no
  "\x"        "\x00"    no
  "\x7q"      "\x07q"   yes
  "\xq"       "\x00q"   yes

=item [3]

The result is the Unicode character or character sequence given by I<name>.
See L<charnames>.

=item [4]

C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code
point is I<hexadecimal number>.

=item [5]

The character following C<\c> is mapped to some other character as shown in the
table:

 Sequence   Value
   \c@      chr(0)
   \cA      chr(1)
   \ca      chr(1)
   \cB      chr(2)
   \cb      chr(2)
   ...
   \cZ      chr(26)
   \cz      chr(26)
   \c[      chr(27)
   \c]      chr(29)
   \c^      chr(30)
   \c?      chr(127)

Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the
end of a string, because the backslash would be parsed as escaping the end
quote.

On ASCII platforms, the resulting characters from the list above are the
complete set of ASCII controls.  This isn't the case on EBCDIC platforms; see
L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
sequences mean on both ASCII and EBCDIC platforms.

Use of any other character following the "c" besides those listed above is
discouraged, and some are deprecated with the intention of removing
those in Perl 5.16.  What happens for any of these
other characters currently though, is that the value is derived by inverting
the 7th bit (0x40).

To get platform independent controls, you can use C<\N{...}>.

=item [6]

The result is the character specified by the octal number between the braces.
See L</[8]> below for details on which character.

If a character that isn't an octal digit is encountered, a warning is raised,
and the value is based on the octal digits before it, discarding it and all
following characters up to the closing brace.  It is a fatal error if there are
no octal digits at all.

=item [7]

The result is the character specified by the three-digit octal number in the
range 000 to 777 (but best to not use above 077, see next paragraph).  See
L</[8]> below for details on which character.

Some contexts allow 2 or even 1 digit, but any usage without exactly
three digits, the first being a zero, may give unintended results.  (For
example, see L<perlrebackslash/Octal escapes>.)  Starting in Perl 5.14, you may
use C<\o{}> instead, which avoids all these problems.  Otherwise, it is best to
use this construct only for ordinals C<\077> and below, remembering to pad to
the left with zeros to make three digits.  For larger ordinals, either use
C<\o{}> , or convert to something else, such as to hex and use C<\x{}>
instead.

Having fewer than 3 digits may lead to a misleading warning message that says
that what follows is ignored.  For example, C<"\128"> in the ASCII character set
is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
digit '8' ignored> will be thrown.  To avoid this warning, make sure to pad
your octal number with C<0>'s: C<"\0128">.

=item [8]

Several constructs above specify a character by a number.  That number
gives the character's position in the character set encoding (indexed from 0).
This is called synonymously its ordinal, code position, or code point.  Perl
works on platforms that have a native encoding currently of either ASCII/Latin1
or EBCDIC, each of which allow specification of 256 characters.  In general, if
the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
native encoding.  If the number is 256 (0x100, 0400) or above, Perl interprets
it as a Unicode code point and the result is the corresponding Unicode
character.  For example C<\x{50}> and C<\o{120}> both are the number 80 in
decimal, which is less than 256, so the number is interpreted in the native
character set encoding.  In ASCII the character in the 80th position (indexed
from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".
C<\x{100}> and C<\o{400}> are both 256 in decimal, so the number is interpreted
as a Unicode code point no matter what the native encoding is.  The name of the
character in the 100th position (indexed by 0) in Unicode is
C<LATIN CAPITAL LETTER A WITH MACRON>.

There are a couple of exceptions to the above rule.  C<\N{U+I<hex number>}> is
always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even
on EBCDIC platforms.  And if L<C<S<use encoding>>|encoding> is in effect, the
number is considered to be in that encoding, and is translated from that into
the platform's native encoding if there is a corresponding native character;
otherwise to Unicode.

=back

B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>.  (C<\v>
does have meaning in regular expression patterns in Perl, see L<perlre>.)

The following escape sequences are available in constructs that interpolate,
but not in transliterations.
X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>

    \l		lowercase next character only
    \u		titlecase (not uppercase!) next character only
    \L		lowercase all characters till \E seen
    \U		uppercase all characters till \E seen
    \Q		quote non-word characters till \E
    \E		end either case modification or quoted section
		(whichever was last seen)

C<\L>, C<\U>, and C<\Q> can stack, in which case you need one
C<\E> for each.  For example:

	say "This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
    This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?

If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
C<\u>, and C<\U> is taken from the current locale.  See L<perllocale>.
If Unicode (for example, C<\N{}> or code points of 0x100 or
beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and
C<\U> is as defined by Unicode.  That means that case-mapping
a single character can sometimes produce several characters.

All systems use the virtual C<"\n"> to represent a line terminator,
called a "newline".  There is no such thing as an unvarying, physical
newline character.  It is only an illusion that the operating system,
device drivers, C libraries, and Perl all conspire to preserve.  Not all
systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF.  For example,
on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
and on systems without line terminator,
printing C<"\n"> might emit no actual data.  In general, use C<"\n"> when
you mean a "newline" for your system, but use the literal ASCII when you
need an exact character.  For example, most networking protocols expect
and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
and although they often accept just C<"\012">, they seldom tolerate just
C<"\015">.  If you get in the habit of using C<"\n"> for networking,
you may be burned some day.
X<newline> X<line terminator> X<eol> X<end of line>
X<\n> X<\r> X<\r\n>

For constructs that do interpolate, variables beginning with "C<$>"
or "C<@>" are interpolated.  Subscripted variables such as C<$a[3]> or
C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
But method calls such as C<< $obj->meth >> are not.

Interpolating an array or slice interpolates the elements in order,
separated by the value of C<$">, so is equivalent to interpolating
C<join $", @array>.  "Punctuation" arrays such as C<@*> are usually
interpolated only if the name is enclosed in braces C<@{*}>, but the
arrays C<@_>, C<@+>, and C<@-> are interpolated even without braces.

For double-quoted strings, the quoting from C<\Q> is applied after
interpolation and escapes are processed.

    "abc\Qfoo\tbar$s\Exyz"

is equivalent to

    "abc" . quotemeta("foo\tbar$s") . "xyz"

For the pattern of regex operators (C<qr//>, C<m//> and C<s///>),
the quoting from C<\Q> is applied after interpolation is processed,
but before escapes are processed. This allows the pattern to match
literally (except for C<$> and C<@>). For example, the following matches:

    '\s\t' =~ /\Q\s\t/

Because C<$> or C<@> trigger interpolation, you'll need to use something
like C</\Quser\E\@\Qhost/> to match them literally.

Patterns are subject to an additional level of interpretation as a
regular expression.  This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
pattern from the variables.  If this is not what you want, use C<\Q> to
interpolate a variable literally.

Apart from the behavior described above, Perl does not expand
multiple levels of interpolation.  In particular, contrary to the
expectations of shell programmers, back-quotes do I<NOT> interpolate
within double quotes, nor do single quotes impede evaluation of
variables when used within double quotes.

=head2 Regexp Quote-Like Operators
X<operator, regexp>

Here are the quote-like operators that apply to pattern
matching and related activities.

=over 8

=item qr/STRING/msixpodual
X<qr> X</i> X</m> X</o> X</s> X</x> X</p>

This operator quotes (and possibly compiles) its I<STRING> as a regular
expression.  I<STRING> is interpolated the same way as I<PATTERN>
in C<m/PATTERN/>.  If "'" is used as the delimiter, no interpolation
is done.  Returns a Perl value which may be used instead of the
corresponding C</STRING/msixpodual> expression. The returned value is a
normalized version of the original pattern. It magically differs from
a string containing the same characters: C<ref(qr/x/)> returns "Regexp",
even though dereferencing the result returns undef.

For example,

    $rex = qr/my.STRING/is;
    print $rex;                 # prints (?si-xm:my.STRING)
    s/$rex/foo/;

is equivalent to

    s/my.STRING/foo/is;

The result may be used as a subpattern in a match:

    $re = qr/$pattern/;
    $string =~ /foo${re}bar/;	# can be interpolated in other patterns
    $string =~ $re;		# or used standalone
    $string =~ /$re/;		# or this way

Since Perl may compile the pattern at the moment of execution of the qr()
operator, using qr() may have speed advantages in some situations,
notably if the result of qr() is used standalone:

    sub match {
	my $patterns = shift;
	my @compiled = map qr/$_/i, @$patterns;
	grep {
	    my $success = 0;
	    foreach my $pat (@compiled) {
		$success = 1, last if /$pat/;
	    }
	    $success;
	} @_;
    }

Precompilation of the pattern into an internal representation at
the moment of qr() avoids a need to recompile the pattern every
time a match C</$pat/> is attempted.  (Perl has many other internal
optimizations, but none would be triggered in the above example if
we did not use qr() operator.)

Options (specified by the following modifiers) are:

    m	Treat string as multiple lines.
    s	Treat string as single line. (Make . match a newline)
    i	Do case-insensitive pattern matching.
    x	Use extended regular expressions.
    p	When matching preserve a copy of the matched string so
        that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
    o	Compile pattern only once.
    l   Use the locale
    u   Use Unicode rules
    a   Use ASCII for \d, \s, \w; specifying two a's further restricts
        /i matching so that no ASCII character will match a non-ASCII
        one
    d   Use Unicode or native charset, as in 5.12 and earlier

If a precompiled pattern is embedded in a larger pattern then the effect
of "msixpluad" will be propagated appropriately.  The effect the "o"
modifier has is not propagated, being restricted to those patterns
explicitly using it.

The last four modifiers listed above, added in Perl 5.14,
control the character set semantics.

See L<perlre> for additional information on valid syntax for STRING, and
for a detailed look at the semantics of regular expressions.  In
particular, all the modifiers execpt C</o> are further explained in
L<perlre/Modifiers>.  C</o> is described in the next section.

=item m/PATTERN/msixpodualgc
X<m> X<operator, match>
X<regexp, options> X<regexp> X<regex, options> X<regex>
X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>

=item /PATTERN/msixpodualgc

Searches a string for a pattern match, and in scalar context returns
true if it succeeds, false if it fails.  If no string is specified
via the C<=~> or C<!~> operator, the $_ string is searched.  (The
string specified with C<=~> need not be an lvalue--it may be the
result of an expression evaluation, but remember the C<=~> binds
rather tightly.)  See also L<perlre>.

Options are as described in C<qr//> above; in addition, the following match
process modifiers are available:

 g  Match globally, i.e., find all occurrences.
 c  Do not reset search position on a failed match when /g is in effect.

If "/" is the delimiter then the initial C<m> is optional.  With the C<m>
you can use any pair of non-whitespace (ASCII) characters
as delimiters.  This is particularly useful for matching path names
that contain "/", to avoid LTS (leaning toothpick syndrome).  If "?" is
the delimiter, then a match-only-once rule applies,
described in C<m?PATTERN?> below.
If "'" is the delimiter, no interpolation is performed on the PATTERN.
When using a character valid in an identifier, whitespace is required
after the C<m>.

PATTERN may contain variables, which will be interpolated
every time the pattern search is evaluated, except
for when the delimiter is a single quote.  (Note that C<$(>, C<$)>, and
C<$|> are not interpolated because they look like end-of-string tests.)
Perl will not recompile the pattern unless an interpolated
variable that it contains changes.  You can force Perl to skip the
test and never recompile by adding a C</o> (which stands for "once")
after the trailing delimiter.
Once upon a time, Perl would recompile regular expressions
unnecessarily, and this modifier was useful to tell it not to do so, in the
interests of speed.  But now, the only reasons to use C</o> are either:

=over

=item 1

The variables are thousands of characters long and you know that they
don't change, and you need to wring out the last little bit of speed by
having Perl skip testing for that.  (There is a maintenance penalty for
doing this, as mentioning C</o> constitutes a promise that you won't
change the variables in the pattern.  If you change them, Perl won't
even notice.)

=item 2

you want the pattern to use the initial values of the variables
regardless of whether they change or not.  (But there are saner ways
of accomplishing this than using C</o>.)

=back

=item The empty pattern //

If the PATTERN evaluates to the empty string, the last
I<successfully> matched regular expression is used instead. In this
case, only the C<g> and C<c> flags on the empty pattern are honored;
the other flags are taken from the original pattern. If no match has
previously succeeded, this will (silently) act instead as a genuine
empty pattern (which will always match).

Note that it's possible to confuse Perl into thinking C<//> (the empty
regex) is really C<//> (the defined-or operator).  Perl is usually pretty
good about this, but some pathological cases might trigger this, such as
C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
(C<print $fh(//> or C<print($fh //>?).  In all of these examples, Perl
will assume you meant defined-or.  If you meant the empty regex, just
use parentheses or spaces to disambiguate, or even prefix the empty
regex with an C<m> (so C<//> becomes C<m//>).

=item Matching in list context

If the C</g> option is not used, C<m//> in list context returns a
list consisting of the subexpressions matched by the parentheses in the
pattern, i.e., (C<$1>, C<$2>, C<$3>...).  (Note that here C<$1> etc. are
also set, and that this differs from Perl 4's behavior.)  When there are
no parentheses in the pattern, the return value is the list C<(1)> for
success.  With or without parentheses, an empty list is returned upon
failure.

Examples:

    open(TTY, "+>/dev/tty")
	|| die "can't access /dev/tty: $!";

    <TTY> =~ /^y/i && foo();	# do foo if desired

    if (/Version: *([0-9.]*)/) { $version = $1; }

    next if m#^/usr/spool/uucp#;

    # poor man's grep
    $arg = shift;
    while (<>) {
	print if /$arg/o;	# compile only once (no longer needed!)
    }

    if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))

This last example splits $foo into the first two words and the
remainder of the line, and assigns those three fields to $F1, $F2, and
$Etc.  The conditional is true if any variables were assigned; that is,
if the pattern matched.

The C</g> modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it behaves
depends on the context. In list context, it returns a list of the
substrings matched by any capturing parentheses in the regular
expression. If there are no parentheses, it returns a list of all
the matched strings, as if there were parentheses around the whole
pattern.

In scalar context, each execution of C<m//g> finds the next match,
returning true if it matches, and false if there is no further match.
The position after the last match can be read or set using the C<pos()>
function; see L<perlfunc/pos>. A failed match normally resets the
search position to the beginning of the string, but you can avoid that
by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
string also resets the search position.

=item \G assertion

You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
zero-width assertion that matches the exact position where the
previous C<m//g>, if any, left off. Without the C</g> modifier, the
C<\G> assertion still anchors at C<pos()> as it was at the start of
the operation (see L<perlfunc/pos>), but the match is of course only
attempted once. Using C<\G> without C</g> on a target string that has
not previously had a C</g> match applied to it is the same as using
the C<\A> assertion to match the beginning of the string.  Note also
that, currently, C<\G> is only properly supported when anchored at the
very beginning of the pattern.

Examples:

    # list context
    ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

    # scalar context
    local $/ = "";
    while ($paragraph = <>) {
	while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
	    $sentences++;
	}
    }
    say $sentences;

Here's another way to check for sentences in a paragraph:

    my $sentence_rx = qr{
	(?: (?<= ^ ) | (?<= \s ) )  # after start-of-string or whitespace
	\p{Lu}                      # capital letter
	.*?                         # a bunch of anything
	(?<= \S )                   # that ends in non-whitespace
	(?<! \b [DMS]r  )           # but isn't a common abbreviation
	(?<! \b Mrs )
	(?<! \b Sra )
	(?<! \b St  )
	[.?!]                       # followed by a sentence ender
	(?= $ | \s )                # in front of end-of-string or whitespace
    }sx;
    local $/ = "";
    while (my $paragraph = <>) {
	say "NEW PARAGRAPH";
	my $count = 0;
	while ($paragraph =~ /($sentence_rx)/g) {
	    printf "\tgot sentence %d: <%s>\n", ++$count, $1;
	}
    }

Here's how to use C<m//gc> with C<\G>:

    $_ = "ppooqppqq";
    while ($i++ < 2) {
        print "1: '";
        print $1 while /(o)/gc; print "', pos=", pos, "\n";
        print "2: '";
        print $1 if /\G(q)/gc;  print "', pos=", pos, "\n";
        print "3: '";
        print $1 while /(p)/gc; print "', pos=", pos, "\n";
    }
    print "Final: '$1', pos=",pos,"\n" if /\G(.)/;

The last example should print:

    1: 'oo', pos=4
    2: 'q', pos=5
    3: 'pp', pos=7
    1: '', pos=7
    2: 'q', pos=8
    3: '', pos=8
    Final: 'q', pos=8

Notice that the final match matched C<q> instead of C<p>, which a match
without the C<\G> anchor would have done. Also note that the final match
did not update C<pos>. C<pos> is only updated on a C</g> match. If the
final match did indeed match C<p>, it's a good bet that you're running a
very old (pre-5.6.0) version of Perl.

A useful idiom for C<lex>-like scanners is C</\G.../gc>.  You can
combine several regexps like this to process a string part-by-part,
doing different actions depending on which regexp matched.  Each
regexp tries to match where the previous one leaves off.

 $_ = <<'EOL';
    $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
 EOL

 LOOP: {
     print(" digits"),       redo LOOP if /\G\d+\b[,.;]?\s*/gc;
     print(" lowercase"),    redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc;
     print(" UPPERCASE"),    redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc;
     print(" Capitalized"),  redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
     print(" MiXeD"),        redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
     print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
     print(" line-noise"),   redo LOOP if /\G\W+/gc;
     print ". That's all!\n";
 }

Here is the output (split into several lines):

    line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
    line-noise lowercase line-noise lowercase line-noise lowercase
    lowercase line-noise lowercase lowercase line-noise lowercase
    lowercase line-noise MiXeD line-noise. That's all!

=item m?PATTERN?msixpodualgc
X<?> X<operator, match-once>

=item ?PATTERN?msixpodualgc

This is just like the C<m/PATTERN/> search, except that it matches
only once between calls to the reset() operator.  This is a useful
optimization when you want to see only the first occurrence of
something in each file of a set of files, for instance.  Only C<m??>
patterns local to the current package are reset.

    while (<>) {
	if (m?^$?) {
			    # blank line between header and body
	}
    } continue {
	reset if eof;	    # clear m?? status for next file
    }

Another example switched the first "latin1" encoding it finds
to "utf8" in a pod file:

    s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;

The match-once behavior is controlled by the match delimiter being
C<?>; with any other delimiter this is the normal C<m//> operator.  

For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
but the resulting C<?PATTERN?> syntax is deprecated, will warn on
usage and might be removed from a future stable release of Perl (without
further notice!).

=item s/PATTERN/REPLACEMENT/msixpodualgcer
X<substitute> X<substitution> X<replace> X<regexp, replace>
X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r>

Searches a string for a pattern, and if found, replaces that pattern
with the replacement text and returns the number of substitutions
made.  Otherwise it returns false (specifically, the empty string).

If the C</r> (non-destructive) option is used then it runs the
substitution on a copy of the string and instead of returning the
number of substitutions, it returns the copy whether or not a
substitution occurred.  The original string is never changed when
C</r> is used.  The copy will always be a plain string, even if the
input is an object or a tied variable.

If no string is specified via the C<=~> or C<!~> operator, the C<$_>
variable is searched and modified.  Unless the C</r> option is used,
the string specified must be a scalar variable, an array element, a
hash element, or an assignment to one of those; that is, some sort of
scalar lvalue.

If the delimiter chosen is a single quote, no interpolation is
done on either the PATTERN or the REPLACEMENT.  Otherwise, if the
PATTERN contains a $ that looks like a variable rather than an
end-of-string test, the variable will be interpolated into the pattern
at run-time.  If you want the pattern compiled only once the first time
the variable is interpolated, use the C</o> option.  If the pattern
evaluates to the empty string, the last successfully executed regular
expression is used instead.  See L<perlre> for further explanation on these.

Options are as with m// with the addition of the following replacement
specific options:

    e	Evaluate the right side as an expression.
    ee  Evaluate the right side as a string then eval the result.
    r   Return substitution and leave the original string untouched.

Any non-whitespace delimiter may replace the slashes.  Add space after
the C<s> when using a character allowed in identifiers.  If single quotes
are used, no interpretation is done on the replacement string (the C</e>
modifier overrides this, however).  Unlike Perl 4, Perl 5 treats backticks
as normal delimiters; the replacement text is not evaluated as a command.
If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has
its own pair of quotes, which may or may not be bracketing quotes, e.g.,
C<s(foo)(bar)> or C<< s<foo>/bar/ >>.  A C</e> will cause the
replacement portion to be treated as a full-fledged Perl expression
and evaluated right then and there.  It is, however, syntax checked at
compile-time. A second C<e> modifier will cause the replacement portion
to be C<eval>ed before being run as a Perl expression.

Examples:

    s/\bgreen\b/mauve/g;		# don't change wintergreen

    $path =~ s|/usr/bin|/usr/local/bin|;

    s/Login: $foo/Login: $bar/; # run-time pattern

    ($foo = $bar) =~ s/this/that/;	# copy first, then change
    ($foo = "$bar") =~ s/this/that/;	# convert to string, copy, then change
    $foo = $bar =~ s/this/that/r;	# Same as above using /r
    $foo = $bar =~ s/this/that/r
                =~ s/that/the other/r;	# Chained substitutes using /r
    @foo = map { s/this/that/r } @bar	# /r is very useful in maps

    $count = ($paragraph =~ s/Mister\b/Mr./g);  # get change-count

    $_ = 'abc123xyz';
    s/\d+/$&*2/e;		# yields 'abc246xyz'
    s/\d+/sprintf("%5d",$&)/e;	# yields 'abc  246xyz'
    s/\w/$& x 2/eg;		# yields 'aabbcc  224466xxyyzz'

    s/%(.)/$percent{$1}/g;	# change percent escapes; no /e
    s/%(.)/$percent{$1} || $&/ge;	# expr now, so /e
    s/^=(\w+)/pod($1)/ge;	# use function call

    $_ = 'abc123xyz';
    $a = s/abc/def/r;           # $a is 'def123xyz' and
                                # $_ remains 'abc123xyz'.

    # expand variables in $_, but dynamics only, using
    # symbolic dereferencing
    s/\$(\w+)/${$1}/g;

    # Add one to the value of any numbers in the string
    s/(\d+)/1 + $1/eg;

    # Titlecase words in the last 30 characters only
    substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;

    # This will expand any embedded scalar variable
    # (including lexicals) in $_ : First $1 is interpolated
    # to the variable name, and then evaluated
    s/(\$\w+)/$1/eeg;

    # Delete (most) C comments.
    $program =~ s {
	/\*	# Match the opening delimiter.
	.*?	# Match a minimal number of characters.
	\*/	# Match the closing delimiter.
    } []gsx;

    s/^\s*(.*?)\s*$/$1/;	# trim whitespace in $_, expensively

    for ($variable) {		# trim whitespace in $variable, cheap
	s/^\s+//;
	s/\s+$//;
    }

    s/([^ ]*) *([^ ]*)/$2 $1/;	# reverse 1st two fields

Note the use of $ instead of \ in the last example.  Unlike
B<sed>, we use the \<I<digit>> form in only the left hand side.
Anywhere else it's $<I<digit>>.

Occasionally, you can't use just a C</g> to get all the changes
to occur that you might want.  Here are two common cases:

    # put commas in the right places in an integer
    1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;

    # expand tabs to 8-column spacing
    1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;

C<s///le> is treated as a substitution followed by the C<le> operator, not
the C</le> flags.  This may change in a future version of Perl.  It
produces a warning if warnings are enabled.  To disambiguate, use a space
or change the order of the flags:

    s/foo/bar/ le 5;  # "le" infix operator
    s/foo/bar/el;     # "e" and "l" flags

=back

=head2 Quote-Like Operators
X<operator, quote-like>

=over 4

=item q/STRING/
X<q> X<quote, single> X<'> X<''>

=item 'STRING'

A single-quoted, literal string.  A backslash represents a backslash
unless followed by the delimiter or another backslash, in which case
the delimiter or backslash is interpolated.

    $foo = q!I said, "You said, 'She said it.'"!;
    $bar = q('This is it.');
    $baz = '\n';		# a two-character string

=item qq/STRING/
X<qq> X<quote, double> X<"> X<"">

=item "STRING"

A double-quoted, interpolated string.

    $_ .= qq
     (*** The previous line contains the naughty word "$1".\n)
		if /\b(tcl|java|python)\b/i;      # :-)
    $baz = "\n";		# a one-character string

=item qx/STRING/
X<qx> X<`> X<``> X<backtick>

=item `STRING`

A string which is (possibly) interpolated and then executed as a
system command with C</bin/sh> or its equivalent.  Shell wildcards,
pipes, and redirections will be honored.  The collected standard
output of the command is returned; standard error is unaffected.  In
scalar context, it comes back as a single (potentially multi-line)
string, or undef if the command failed.  In list context, returns a
list of lines (however you've defined lines with $/ or
$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.

Because backticks do not affect standard error, use shell file descriptor
syntax (assuming the shell supports this) if you care to address this.
To capture a command's STDERR and STDOUT together:

    $output = `cmd 2>&1`;

To capture a command's STDOUT but discard its STDERR:

    $output = `cmd 2>/dev/null`;

To capture a command's STDERR but discard its STDOUT (ordering is
important here):

    $output = `cmd 2>&1 1>/dev/null`;

To exchange a command's STDOUT and STDERR in order to capture the STDERR
but leave its STDOUT to come out the old STDERR:

    $output = `cmd 3>&1 1>&2 2>&3 3>&-`;

To read both a command's STDOUT and its STDERR separately, it's easiest
to redirect them separately to files, and then read from those files
when the program is done:

    system("program args 1>program.stdout 2>program.stderr");

The STDIN filehandle used by the command is inherited from Perl's STDIN.
For example:

    open(SPLAT, "stuff")   || die "can't open stuff: $!";
    open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
    print STDOUT `sort`;

will print the sorted contents of the file named F<"stuff">.

Using single-quote as a delimiter protects the command from Perl's
double-quote interpolation, passing it on to the shell instead:

    $perl_info  = qx(ps $$);            # that's Perl's $$
    $shell_info = qx'ps $$';            # that's the new shell's $$

How that string gets evaluated is entirely subject to the command
interpreter on your system.  On most platforms, you will have to protect
shell metacharacters if you want them treated literally.  This is in
practice difficult to do, as it's unclear how to escape which characters.
See L<perlsec> for a clean and safe example of a manual fork() and exec()
to emulate backticks safely.

On some platforms (notably DOS-like ones), the shell may not be
capable of dealing with multiline commands, so putting newlines in
the string may not get you what you want.  You may be able to evaluate
multiple commands in a single line by separating them with the command
separator character, if your shell supports that (e.g. C<;> on many Unix
shells; C<&> on the Windows NT C<cmd> shell).

Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before starting the child process, but this may not be supported
on some platforms (see L<perlport>).  To be safe, you may need to set
C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
C<IO::Handle> on any open handles.

Beware that some command shells may place restrictions on the length
of the command line.  You must ensure your strings don't exceed this
limit after any necessary interpolations.  See the platform-specific
release notes for more details about your particular environment.

Using this operator can lead to programs that are difficult to port,
because the shell commands called vary between systems, and may in
fact not be present at all.  As one example, the C<type> command under
the POSIX shell is very different from the C<type> command under DOS.
That doesn't mean you should go out of your way to avoid backticks
when they're the right way to get something done.  Perl was made to be
a glue language, and one of the things it glues together is commands.
Just understand what you're getting yourself into.

See L</"I/O Operators"> for more discussion.

=item qw/STRING/
X<qw> X<quote, list> X<quote, words>

Evaluates to a list of the words extracted out of STRING, using embedded
whitespace as the word delimiters.  It can be understood as being roughly
equivalent to:

    split(" ", q/STRING/);

the differences being that it generates a real list at compile time, and
in scalar context it returns the last element in the list.  So
this expression:

    qw(foo bar baz)

is semantically equivalent to the list:

    "foo", "bar", "baz"

Some frequently seen examples:

    use POSIX qw( setlocale localeconv )
    @EXPORT = qw( foo bar baz );

A common mistake is to try to separate the words with comma or to
put comments into a multi-line C<qw>-string.  For this reason, the
C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
produces warnings if the STRING contains the "," or the "#" character.

=item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
X<tr> X<y> X<transliterate> X</c> X</d> X</s>

=item y/SEARCHLIST/REPLACEMENTLIST/cdsr

Transliterates all occurrences of the characters found in the search list
with the corresponding character in the replacement list.  It returns
the number of characters replaced or deleted.  If no string is
specified via the C<=~> or C<!~> operator, the $_ string is transliterated.

If the C</r> (non-destructive) option is present, a new copy of the string
is made and its characters transliterated, and this copy is returned no
matter whether it was modified or not: the original string is always
left unchanged.  The new copy is always a plain string, even if the input
string is an object or a tied variable.

Unless the C</r> option is used, the string specified with C<=~> must be a
scalar variable, an array element, a hash element, or an assignment to one
of those; in other words, an lvalue.

A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
For B<sed> devotees, C<y> is provided as a synonym for C<tr>.  If the
SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
its own pair of quotes, which may or may not be bracketing quotes;
for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.

Note that C<tr> does B<not> do regular expression character classes such as
C<\d> or C<\pL>.  The C<tr> operator is not equivalent to the tr(1)
utility.  If you want to map strings between lower/upper cases, see
L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s>
operator if you need regular expressions.  The C<\U>, C<\u>, C<\L>, and
C<\l> string-interpolation escapes on the right side of a substitution
operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
(except sometimes on legacy 7-bit data).

Note also that the whole range idea is rather unportable between
character sets--and even within character sets they may cause results
you probably didn't expect.  A sound principle is to use only ranges
that begin from and end at either alphabets of equal case (a-e, A-E),
or digits (0-4).  Anything else is unsafe.  If in doubt, spell out the
character sets in full.

Options:

    c	Complement the SEARCHLIST.
    d	Delete found but unreplaced characters.
    s	Squash duplicate replaced characters.
    r	Return the modified string and leave the original string
	untouched.

If the C</c> modifier is specified, the SEARCHLIST character set
is complemented.  If the C</d> modifier is specified, any characters
specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
(Note that this is slightly more flexible than the behavior of some
B<tr> programs, which delete anything they find in the SEARCHLIST,
period.) If the C</s> modifier is specified, sequences of characters
that were transliterated to the same character are squashed down
to a single instance of the character.

If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
exactly as specified.  Otherwise, if the REPLACEMENTLIST is shorter
than the SEARCHLIST, the final character is replicated till it is long
enough.  If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
This latter is useful for counting characters in a class or for
squashing character sequences in a class.

Examples:

    $ARGV[1] =~ tr/A-Z/a-z/;	# canonicalize to lower case ASCII

    $cnt = tr/*/*/;		# count the stars in $_

    $cnt = $sky =~ tr/*/*/;	# count the stars in $sky

    $cnt = tr/0-9//;		# count the digits in $_

    tr/a-zA-Z//s;		# bookkeeper -> bokeper

    ($HOST = $host) =~ tr/a-z/A-Z/;
     $HOST = $host  =~ tr/a-z/A-Z/r;   # same thing

    $HOST = $host =~ tr/a-z/A-Z/r    # chained with s///r
                  =~ s/:/ -p/r;

    tr/a-zA-Z/ /cs;		# change non-alphas to single space

    @stripped = map tr/a-zA-Z/ /csr, @original;
				# /r with map

    tr [\200-\377]
       [\000-\177];		# wickedly delete 8th bit

If multiple transliterations are given for a character, only the
first one is used:

    tr/AAA/XYZ/

will transliterate any A to X.

Because the transliteration table is built at compile time, neither
the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
interpolation.  That means that if you want to use variables, you
must use an eval():

    eval "tr/$oldlist/$newlist/";
    die $@ if $@;

    eval "tr/$oldlist/$newlist/, 1" or die $@;

=item <<EOF
X<here-doc> X<heredoc> X<here-document> X<<< << >>>

A line-oriented form of quoting is based on the shell "here-document"
syntax.  Following a C<< << >> you specify a string to terminate
the quoted material, and all lines following the current line down to
the terminating string are the value of the item.

The terminating string may be either an identifier (a word), or some
quoted text.  An unquoted identifier works like double quotes.
There may not be a space between the C<< << >> and the identifier,
unless the identifier is explicitly quoted.  (If you put a space it
will be treated as a null identifier, which is valid, and matches the
first empty line.)  The terminating string must appear by itself
(unquoted and with no surrounding whitespace) on the terminating line.

If the terminating string is quoted, the type of quotes used determine
the treatment of the text.

=over 4

=item Double Quotes

Double quotes indicate that the text will be interpolated using exactly
the same rules as normal double quoted strings.

       print <<EOF;
    The price is $Price.
    EOF

       print << "EOF"; # same as above
    The price is $Price.
    EOF


=item Single Quotes

Single quotes indicate the text is to be treated literally with no
interpolation of its content. This is similar to single quoted
strings except that backslashes have no special meaning, with C<\\>
being treated as two backslashes and not one as they would in every
other quoting construct.

Just as in the shell, a backslashed bareword following the C<<< << >>>
means the same thing as a single-quoted string does:

	$cost = <<'VISTA';  # hasta la ...
    That'll be $10 please, ma'am.
    VISTA

	$cost = <<\VISTA;   # Same thing!
    That'll be $10 please, ma'am.
    VISTA

This is the only form of quoting in perl where there is no need
to worry about escaping content, something that code generators
can and do make good use of.

=item Backticks

The content of the here doc is treated just as it would be if the
string were embedded in backticks. Thus the content is interpolated
as though it were double quoted and then executed via the shell, with
the results of the execution returned.

       print << `EOC`; # execute command and get results
    echo hi there
    EOC

=back

It is possible to stack multiple here-docs in a row:

       print <<"foo", <<"bar"; # you can stack them
    I said foo.
    foo
    I said bar.
    bar

       myfunc(<< "THIS", 23, <<'THAT');
    Here's a line
    or two.
    THIS
    and here's another.
    THAT

Just don't forget that you have to put a semicolon on the end
to finish the statement, as Perl doesn't know you're not going to
try to do this:

       print <<ABC
    179231
    ABC
       + 20;

If you want to remove the line terminator from your here-docs,
use C<chomp()>.

    chomp($string = <<'END');
    This is a string.
    END

If you want your here-docs to be indented with the rest of the code,
you'll need to remove leading whitespace from each line manually:

    ($quote = <<'FINIS') =~ s/^\s+//gm;
       The Road goes ever on and on,
       down from the door where it began.
    FINIS

If you use a here-doc within a delimited construct, such as in C<s///eg>,
the quoted material must come on the lines following the final delimiter.
So instead of

    s/this/<<E . 'that'
    the other
    E
     . 'more '/eg;

you have to write

    s/this/<<E . 'that'
     . 'more '/eg;
    the other
    E

If the terminating identifier is on the last line of the program, you
must be sure there is a newline after it; otherwise, Perl will give the
warning B<Can't find string terminator "END" anywhere before EOF...>.

Additionally, quoting rules for the end-of-string identifier are
unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
supported in place of C<''> and C<"">, and the only interpolation is for
backslashing the quoting character:

    print << "abc\"def";
    testing...
    abc"def

Finally, quoted strings cannot span multiple lines.  The general rule is
that the identifier must be a string literal.  Stick with that, and you
should be safe.

=back

=head2 Gory details of parsing quoted constructs
X<quote, gory details>

When presented with something that might have several different
interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
principle to pick the most probable interpretation.  This strategy
is so successful that Perl programmers often do not suspect the
ambivalence of what they write.  But from time to time, Perl's
notions differ substantially from what the author honestly meant.

This section hopes to clarify how Perl handles quoted constructs.
Although the most common reason to learn this is to unravel labyrinthine
regular expressions, because the initial steps of parsing are the
same for all quoting operators, they are all discussed together.

The most important Perl parsing rule is the first one discussed
below: when processing a quoted construct, Perl first finds the end
of that construct, then interprets its contents.  If you understand
this rule, you may skip the rest of this section on the first
reading.  The other rules are likely to contradict the user's
expectations much less frequently than this first one.

Some passes discussed below are performed concurrently, but because
their results are the same, we consider them individually.  For different
quoting constructs, Perl performs different numbers of passes, from
one to four, but these passes are always performed in the same order.

=over 4

=item Finding the end

The first pass is finding the end of the quoted construct, where
the information about the delimiters is used in parsing.
During this search, text between the starting and ending delimiters
is copied to a safe location. The text copied gets delimiter-independent.

If the construct is a here-doc, the ending delimiter is a line
that has a terminating string as the content. Therefore C<<<EOF> is
terminated by C<EOF> immediately followed by C<"\n"> and starting
from the first column of the terminating line.
When searching for the terminating line of a here-doc, nothing
is skipped. In other words, lines after the here-doc syntax
are compared with the terminating string line by line.

For the constructs except here-docs, single characters are used as starting
and ending delimiters. If the starting delimiter is an opening punctuation
(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
If the starting delimiter is an unpaired character like C</> or a closing
punctuation, the ending delimiter is same as the starting delimiter.
Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
C<qq[]> and C<qq]]> constructs.

When searching for single-character delimiters, escaped delimiters
and C<\\> are skipped. For example, while searching for terminating C</>,
combinations of C<\\> and C<\/> are skipped.  If the delimiters are
bracketing, nested pairs are also skipped.  For example, while searching
for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>,
and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well.
However, when backslashes are used as the delimiters (like C<qq\\> and
C<tr\\\>), nothing is skipped.
During the search for the end, backslashes that escape delimiters
are removed (exactly speaking, they are not copied to the safe location).

For constructs with three-part delimiters (C<s///>, C<y///>, and
C<tr///>), the search is repeated once more.
If the first delimiter is not an opening punctuation, three delimiters must
be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
terminates the left part and starts the right part at once.
If the left part is delimited by bracketing punctuation (that is C<()>,
C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
delimiters such as C<s(){}> and C<tr[]//>.  In these cases, whitespace
and comments are allowed between both parts, though the comment must follow
at least one whitespace character; otherwise a character expected as the 
start of the comment may be regarded as the starting delimiter of the right part.

During this search no attention is paid to the semantics of the construct.
Thus:

    "$hash{"$foo/$bar"}"

or:

    m/
      bar	# NOT a comment, this slash / terminated m//!
     /x

do not form legal quoted expressions.   The quoted part ends on the
first C<"> and C</>, and the rest happens to be a syntax error.
Because the slash that terminated C<m//> was followed by a C<SPACE>,
the example above is not C<m//x>, but rather C<m//> with no C</x>
modifier.  So the embedded C<#> is interpreted as a literal C<#>.

Also no attention is paid to C<\c\> (multichar control char syntax) during
this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
of C<\/>, and the following C</> is not recognized as a delimiter.
Instead, use C<\034> or C<\x1c> at the end of quoted constructs.

=item Interpolation
X<interpolation>

The next step is interpolation in the text obtained, which is now
delimiter-independent.  There are multiple cases.

=over 4

=item C<<<'EOF'>

No interpolation is performed.
Note that the combination C<\\> is left intact, since escaped delimiters
are not available for here-docs.

=item  C<m''>, the pattern of C<s'''>

No interpolation is performed at this stage.
Any backslashed sequences including C<\\> are treated at the stage
to L</"parsing regular expressions">.

=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>

The only interpolation is removal of C<\> from pairs of C<\\>.
Therefore C<-> in C<tr'''> and C<y'''> is treated literally
as a hyphen and no character range is available.
C<\1> in the replacement of C<s'''> does not work as C<$1>.

=item C<tr///>, C<y///>

No variable interpolation occurs.  String modifying combinations for
case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
The other escape sequences such as C<\200> and C<\t> and backslashed
characters such as C<\\> and C<\-> are converted to appropriate literals.
The character C<-> is treated specially and therefore C<\-> is treated
as a literal C<->.

=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">

C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
converted to corresponding Perl constructs.  Thus, C<"$foo\Qbaz$bar">
is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
The other escape sequences such as C<\200> and C<\t> and backslashed
characters such as C<\\> and C<\-> are replaced with appropriate
expansions.

Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
is interpolated in the usual way.  Something like C<"\Q\\E"> has
no C<\E> inside.  instead, it has C<\Q>, C<\\>, and C<E>, so the
result is the same as for C<"\\\\E">.  As a general rule, backslashes
between C<\Q> and C<\E> may lead to counterintuitive results.  So,
C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
as C<"\\\t"> (since TAB is not alphanumeric).  Note also that:

  $str = '\t';
  return "\Q$str";

may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.

Interpolated scalars and arrays are converted internally to the C<join> and
C<.> catenation operations.  Thus, C<"$foo XXX '@arr'"> becomes:

  $foo . " XXX '" . (join $", @arr) . "'";

All operations above are performed simultaneously, left to right.

Because the result of C<"\Q STRING \E"> has all metacharacters
quoted, there is no way to insert a literal C<$> or C<@> inside a
C<\Q\E> pair.  If protected by C<\>, C<$> will be quoted to became
C<"\\\$">; if not, it is interpreted as the start of an interpolated
scalar.

Note also that the interpolation code needs to make a decision on
where the interpolated scalar ends.  For instance, whether
C<< "a $b -> {c}" >> really means:

  "a " . $b . " -> {c}";

or:

  "a " . $b -> {c};

Most of the time, the longest possible text that does not include
spaces between components and which contains matching braces or
brackets.  because the outcome may be determined by voting based
on heuristic estimators, the result is not strictly predictable.
Fortunately, it's usually correct for ambiguous cases.

=item the replacement of C<s///>

Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
happens as with C<qq//> constructs.

It is at this step that C<\1> is begrudgingly converted to C<$1> in
the replacement text of C<s///>, in order to correct the incorrigible
I<sed> hackers who haven't picked up the saner idiom yet.  A warning
is emitted if the C<use warnings> pragma or the B<-w> command-line flag
(that is, the C<$^W> variable) was set.

=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,

Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
and interpolation happens (almost) as with C<qq//> constructs.

Processing of C<\N{...}> is also done here, and compiled into an intermediate
form for the regex compiler.  (This is because, as mentioned below, the regex
compilation may be done at execution time, and C<\N{...}> is a compile-time
construct.)

However any other combinations of C<\> followed by a character
are not substituted but only skipped, in order to parse them
as regular expressions at the following step.
As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
treated as an array symbol (for example C<@foo>),
even though the same text in C<qq//> gives interpolation of C<\c@>.

Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and
a C<#>-comment in a C<//x>-regular expression, no processing is
performed whatsoever.  This is the first step at which the presence
of the C<//x> modifier is relevant.

Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
voted (by several different estimators) to be either an array element
or C<$var> followed by an RE alternative.  This is where the notation
C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
array element C<-9>, not as a regular expression from the variable
C<$arr> followed by a digit, which would be the interpretation of
C</$arr[0-9]/>.  Since voting among different estimators may occur,
the result is not predictable.

The lack of processing of C<\\> creates specific restrictions on
the post-processed text.  If the delimiter is C</>, one cannot get
the combination C<\/> into the result of this step.  C</> will
finish the regular expression, C<\/> will be stripped to C</> on
the previous step, and C<\\/> will be left as is.  Because C</> is
equivalent to C<\/> inside a regular expression, this does not
matter unless the delimiter happens to be character special to the
RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
alphanumeric char, as in:

  m m ^ a \s* b mmx;

In the RE above, which is intentionally obfuscated for illustration, the
delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
RE is the same as for C<m/ ^ a \s* b /mx>.  There's more than one
reason you're encouraged to restrict your delimiters to non-alphanumeric,
non-whitespace choices.

=back

This step is the last one for all constructs except regular expressions,
which are processed further.

=item parsing regular expressions
X<regexp, parse>

Previous steps were performed during the compilation of Perl code,
but this one happens at run time, although it may be optimized to
be calculated at compile time if appropriate.  After preprocessing
described above, and possibly after evaluation if concatenation,
joining, casing translation, or metaquoting are involved, the
resulting I<string> is passed to the RE engine for compilation.

Whatever happens in the RE engine might be better discussed in L<perlre>,
but for the sake of continuity, we shall do so here.

This is another step where the presence of the C<//x> modifier is
relevant.  The RE engine scans the string from left to right and
converts it to a finite automaton.

Backslashed characters are either replaced with corresponding
literal strings (as with C<\{>), or else they generate special nodes
in the finite automaton (as with C<\b>).  Characters special to the
RE engine (such as C<|>) generate corresponding nodes or groups of
nodes.  C<(?#...)> comments are ignored.  All the rest is either
converted to literal strings to match, or else is ignored (as is
whitespace and C<#>-style comments if C<//x> is present).

Parsing of the bracketed character class construct, C<[...]>, is
rather different than the rule used for the rest of the pattern.
The terminator of this construct is found using the same rules as
for finding the terminator of a C<{}>-delimited construct, the only
exception being that C<]> immediately following C<[> is treated as
though preceded by a backslash.  Similarly, the terminator of
C<(?{...})> is found using the same rules as for finding the
terminator of a C<{}>-delimited construct.

It is possible to inspect both the string given to RE engine and the
resulting finite automaton.  See the arguments C<debug>/C<debugcolor>
in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
switch documented in L<perlrun/"Command Switches">.

=item Optimization of regular expressions
X<regexp, optimization>

This step is listed for completeness only.  Since it does not change
semantics, details of this step are not documented and are subject
to change without notice.  This step is performed over the finite
automaton that was generated during the previous pass.

It is at this stage that C<split()> silently optimizes C</^/> to
mean C</^/m>.

=back

=head2 I/O Operators
X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
X<< <> >> X<@ARGV>

There are several I/O operators you should know about.

A string enclosed by backticks (grave accents) first undergoes
double-quote interpolation.  It is then interpreted as an external
command, and the output of that command is the value of the
backtick string, like in a shell.  In scalar context, a single string
consisting of all output is returned.  In list context, a list of
values is returned, one per line of output.  (You can set C<$/> to use
a different line terminator.)  The command is executed each time the
pseudo-literal is evaluated.  The status value of the command is
returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
Unlike in B<csh>, no translation is done on the return data--newlines
remain newlines.  Unlike in any of the shells, single quotes do not
hide variable names in the command from interpretation.  To pass a
literal dollar-sign through to the shell you need to hide it with a
backslash.  The generalized form of backticks is C<qx//>.  (Because
backticks always undergo shell expansion as well, see L<perlsec> for
security concerns.)
X<qx> X<`> X<``> X<backtick> X<glob>

In scalar context, evaluating a filehandle in angle brackets yields
the next line from that file (the newline, if any, included), or
C<undef> at end-of-file or on error.  When C<$/> is set to C<undef>
(sometimes known as file-slurp mode) and the file is empty, it
returns C<''> the first time, followed by C<undef> subsequently.

Ordinarily you must assign the returned value to a variable, but
there is one situation where an automatic assignment happens.  If
and only if the input symbol is the only thing inside the conditional
of a C<while> statement (even if disguised as a C<for(;;)> loop),
the value is automatically assigned to the global variable $_,
destroying whatever was there previously.  (This may seem like an
odd thing to you, but you'll use the construct in almost every Perl
script you write.)  The $_ variable is not implicitly localized.
You'll have to put a C<local $_;> before the loop if you want that
to happen.

The following lines are equivalent:

    while (defined($_ = <STDIN>)) { print; }
    while ($_ = <STDIN>) { print; }
    while (<STDIN>) { print; }
    for (;<STDIN>;) { print; }
    print while defined($_ = <STDIN>);
    print while ($_ = <STDIN>);
    print while <STDIN>;

This also behaves similarly, but avoids $_ :

    while (my $line = <STDIN>) { print $line }

In these loop constructs, the assigned value (whether assignment
is automatic or explicit) is then tested to see whether it is
defined.  The defined test avoids problems where line has a string
value that would be treated as false by Perl, for example a "" or
a "0" with no trailing newline.  If you really mean for such values
to terminate the loop, they should be tested for explicitly:

    while (($_ = <STDIN>) ne '0') { ... }
    while (<STDIN>) { last unless $_; ... }

In other boolean contexts, C<< <filehandle> >> without an
explicit C<defined> test or comparison elicits a warning if the
C<use warnings> pragma or the B<-w>
command-line switch (the C<$^W> variable) is in effect.

The filehandles STDIN, STDOUT, and STDERR are predefined.  (The
filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
in packages, where they would be interpreted as local identifiers
rather than global.)  Additional filehandles may be created with
the open() function, amongst others.  See L<perlopentut> and
L<perlfunc/open> for details on this.
X<stdin> X<stdout> X<sterr>

If a <FILEHANDLE> is used in a context that is looking for
a list, a list comprising all input lines is returned, one line per
list element.  It's easy to grow to a rather large data space this
way, so use with care.

<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
See L<perlfunc/readline>.

The null filehandle <> is special: it can be used to emulate the
behavior of B<sed> and B<awk>.  Input from <> comes either from
standard input, or from each file listed on the command line.  Here's
how it works: the first time <> is evaluated, the @ARGV array is
checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
gives you standard input.  The @ARGV array is then processed as a list
of filenames.  The loop

    while (<>) {
	...			# code for each line
    }

is equivalent to the following Perl-like pseudo code:

    unshift(@ARGV, '-') unless @ARGV;
    while ($ARGV = shift) {
	open(ARGV, $ARGV);
	while (<ARGV>) {
	    ...		# code for each line
	}
    }

except that it isn't so cumbersome to say, and will actually work.
It really does shift the @ARGV array and put the current filename
into the $ARGV variable.  It also uses filehandle I<ARGV>
internally. <> is just a synonym for <ARGV>, which
is magical.  (The pseudo code above doesn't work because it treats
<ARGV> as non-magical.)

Since the null filehandle uses the two argument form of L<perlfunc/open>
it interprets special characters, so if you have a script like this:

    while (<>) {
        print;
    }

and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
If you want all items in C<@ARGV> to be interpreted as file names, you
can use the module C<ARGV::readonly> from CPAN.

You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want.  Line numbers (C<$.>)
continue as though the input were one big happy file.  See the example
in L<perlfunc/eof> for how to reset line numbers on each file.

If you want to set @ARGV to your own list of files, go right ahead.
This sets @ARGV to all plain text files if no @ARGV was given:

    @ARGV = grep { -f && -T } glob('*') unless @ARGV;

You can even set them to pipe commands.  For example, this automatically
filters compressed arguments through B<gzip>:

    @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;

If you want to pass switches into your script, you can use one of the
Getopts modules or put a loop on the front like this:

    while ($_ = $ARGV[0], /^-/) {
	shift;
        last if /^--$/;
	if (/^-D(.*)/) { $debug = $1 }
	if (/^-v/)     { $verbose++  }
	# ...		# other switches
    }

    while (<>) {
	# ...		# code for each line
    }

The <> symbol will return C<undef> for end-of-file only once.
If you call it again after this, it will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will read input from STDIN.

If what the angle brackets contain is a simple scalar variable (e.g.,
<$foo>), then that variable contains the name of the
filehandle to input from, or its typeglob, or a reference to the
same.  For example:

    $fh = \*STDIN;
    $line = <$fh>;

If what's within the angle brackets is neither a filehandle nor a simple
scalar variable containing a filehandle name, typeglob, or typeglob
reference, it is interpreted as a filename pattern to be globbed, and
either a list of filenames or the next filename in the list is returned,
depending on context.  This distinction is determined on syntactic
grounds alone.  That means C<< <$x> >> is always a readline() from
an indirect handle, but C<< <$hash{key}> >> is always a glob().
That's because $x is a simple scalar variable, but C<$hash{key}> is
not--it's a hash element.  Even C<< <$x > >> (note the extra space)
is treated as C<glob("$x ")>, not C<readline($x)>.

One level of double-quote interpretation is done first, but you can't
say C<< <$foo> >> because that's an indirect filehandle as explained
in the previous paragraph.  (In older versions of Perl, programmers
would insert curly brackets to force interpretation as a filename glob:
C<< <${foo}> >>.  These days, it's considered cleaner to call the
internal function directly as C<glob($foo)>, which is probably the right
way to have done it in the first place.)  For example:

    while (<*.c>) {
	chmod 0644, $_;
    }

is roughly equivalent to:

    open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
    while (<FOO>) {
	chomp;
	chmod 0644, $_;
    }

except that the globbing is actually done internally using the standard
C<File::Glob> extension.  Of course, the shortest way to do the above is:

    chmod 0644, <*.c>;

A (file)glob evaluates its (embedded) argument only when it is
starting a new list.  All values must be read before it will start
over.  In list context, this isn't important because you automatically
get them all anyway.  However, in scalar context the operator returns
the next value each time it's called, or C<undef> when the list has
run out.  As with filehandle reads, an automatic C<defined> is
generated when the glob occurs in the test part of a C<while>,
because legal glob returns (e.g. a file called F<0>) would otherwise
terminate the loop.  Again, C<undef> is returned only once.  So if
you're expecting a single value from a glob, it is much better to
say

    ($file) = <blurch*>;

than

    $file = <blurch*>;

because the latter will alternate between returning a filename and
returning false.

If you're trying to do variable interpolation, it's definitely better
to use the glob() function, because the older notation can cause people
to become confused with the indirect filehandle notation.

    @files = glob("$dir/*.[ch]");
    @files = glob($files[$i]);

=head2 Constant Folding
X<constant folding> X<folding>

Like C, Perl does a certain amount of expression evaluation at
compile time whenever it determines that all arguments to an
operator are static and have no side effects.  In particular, string
concatenation happens at compile time between literals that don't do
variable substitution.  Backslash interpolation also happens at
compile time.  You can say

    'Now is the time for all' . "\n" .
	'good men to come to.'

and this all reduces to one string internally.  Likewise, if
you say

    foreach $file (@filenames) {
	if (-s $file > 5 + 100 * 2**16) {  }
    }

the compiler will precompute the number which that expression
represents so that the interpreter won't have to.

=head2 No-ops
X<no-op> X<nop>

Perl doesn't officially have a no-op operator, but the bare constants
C<0> and C<1> are special-cased to not produce a warning in a void
context, so you can for example safely do

    1 while foo();

=head2 Bitwise String Operators
X<operator, bitwise, string>

Bitstrings of any size may be manipulated by the bitwise operators
(C<~ | & ^>).

If the operands to a binary bitwise op are strings of different
sizes, B<|> and B<^> ops act as though the shorter operand had
additional zero bits on the right, while the B<&> op acts as though
the longer operand were truncated to the length of the shorter.
The granularity for such extension or truncation is one or more
bytes.

    # ASCII-based examples
    print "j p \n" ^ " a h";        	# prints "JAPH\n"
    print "JA" | "  ph\n";          	# prints "japh\n"
    print "japh\nJunk" & '_____';   	# prints "JAPH\n";
    print 'p N$' ^ " E<H\n";		# prints "Perl\n";

If you are intending to manipulate bitstrings, be certain that
you're supplying bitstrings: If an operand is a number, that will imply
a B<numeric> bitwise operation.  You may explicitly show which type of
operation you intend by using C<""> or C<0+>, as in the examples below.

    $foo =  150  |  105;	# yields 255  (0x96 | 0x69 is 0xFF)
    $foo = '150' |  105;	# yields 255
    $foo =  150  | '105';	# yields 255
    $foo = '150' | '105';	# yields string '155' (under ASCII)

    $baz = 0+$foo & 0+$bar;	# both ops explicitly numeric
    $biz = "$foo" ^ "$bar";	# both ops explicitly stringy

See L<perlfunc/vec> for information on how to manipulate individual bits
in a bit vector.

=head2 Integer Arithmetic
X<integer>

By default, Perl assumes that it must do most of its arithmetic in
floating point.  But by saying

    use integer;

you may tell the compiler to use integer operations
(see L<integer> for a detailed explanation) from here to the end of
the enclosing BLOCK.  An inner BLOCK may countermand this by saying

    no integer;

which lasts until the end of that BLOCK.  Note that this doesn't
mean everything is an integer, merely that Perl will use integer
operations for arithmetic, comparison, and bitwise operators.  For
example, even under C<use integer>, if you take the C<sqrt(2)>, you'll
still get C<1.4142135623731> or so.

Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
and ">>") always produce integral results.  (But see also
L<Bitwise String Operators>.)  However, C<use integer> still has meaning for
them.  By default, their results are interpreted as unsigned integers, but
if C<use integer> is in effect, their results are interpreted
as signed integers.  For example, C<~0> usually evaluates to a large
integral value.  However, C<use integer; ~0> is C<-1> on two's-complement
machines.

=head2 Floating-point Arithmetic
X<floating-point> X<floating point> X<float> X<real>

While C<use integer> provides integer-only arithmetic, there is no
analogous mechanism to provide automatic rounding or truncation to a
certain number of decimal places.  For rounding to a certain number
of digits, sprintf() or printf() is usually the easiest route.
See L<perlfaq4>.

Floating-point numbers are only approximations to what a mathematician
would call real numbers.  There are infinitely more reals than floats,
so some corners must be cut.  For example:

    printf "%.20g\n", 123456789123456789;
    #        produces 123456789123456784

Testing for exact floating-point equality or inequality is not a
good idea.  Here's a (relatively expensive) work-around to compare
whether two floating-point numbers are equal to a particular number of
decimal places.  See Knuth, volume II, for a more robust treatment of
this topic.

    sub fp_equal {
	my ($X, $Y, $POINTS) = @_;
	my ($tX, $tY);
	$tX = sprintf("%.${POINTS}g", $X);
	$tY = sprintf("%.${POINTS}g", $Y);
	return $tX eq $tY;
    }

The POSIX module (part of the standard perl distribution) implements
ceil(), floor(), and other mathematical and trigonometric functions.
The Math::Complex module (part of the standard perl distribution)
defines mathematical functions that work on both the reals and the
imaginary numbers.  Math::Complex not as efficient as POSIX, but
POSIX can't work with complex numbers.

Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely.  In these
cases, it probably pays not to trust whichever system rounding is
being used by Perl, but to instead implement the rounding function you
need yourself.

=head2 Bigger Numbers
X<number, arbitrary precision>

The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules,
along with the C<bigint>, C<bigrat>, and C<bitfloat> pragmas, provide
variable-precision arithmetic and overloaded operators, although
they're currently pretty slow. At the cost of some space and
considerable speed, they avoid the normal pitfalls associated with
limited-precision representations.

	use 5.010;
	use bigint;  # easy interface to Math::BigInt
	$x = 123456789123456789;
	say $x * $x;
    +15241578780673678515622620750190521

Or with rationals:

	use 5.010;
	use bigrat;
	$a = 3/22;
	$b = 4/6;
	say "a/b is ", $a/$b;
	say "a*b is ", $a*$b;
    a/b is 9/44
    a*b is 1/11

Several modules let you calculate with (bound only by memory and CPU time)
unlimited or fixed precision. There are also some non-standard modules that
provide faster implementations via external C libraries.

Here is a short, but incomplete summary:

  Math::Fraction         big, unlimited fractions like 9973 / 12967
  Math::String           treat string sequences like numbers
  Math::FixedPrecision   calculate with a fixed precision
  Math::Currency         for currency calculations
  Bit::Vector            manipulate bit vectors fast (uses C)
  Math::BigIntFast       Bit::Vector wrapper for big numbers
  Math::Pari             provides access to the Pari C library
  Math::BigInteger       uses an external C library
  Math::Cephes           uses external Cephes C library (no big numbers)
  Math::Cephes::Fraction fractions via the Cephes library
  Math::GMP              another one using an external C library

Choose wisely.

=cut