/etc/mediawiki-extensions/extensions-available/Poem.php is in mediawiki-extensions-base 3.5~deb7u2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/mediawiki-extensions/base/Poem/Poem.php
/usr/share/mediawiki-extensions/base/Poem/Poem.php