/usr/share/perl5/Debian/Debhelper/Sequence/xsf.pm is in xserver-xorg-dev 2:1.19.6-1ubuntu4.
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 | #!/usr/bin/perl
use warnings;
use strict;
use Debian::Debhelper::Dh_Lib;
insert_before("dh_gencontrol", "dh_xsf_substvars");
1;
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