postrm is in libperl-dev 5.26.1-6ubuntu0.3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 | #!/bin/sh
set -e
if dpkg-maintscript-helper supports symlink_to_dir; then
    dpkg-maintscript-helper symlink_to_dir \
        /usr/share/doc/libperl-dev perl 5.22.1-7~ -- "$@"
fi
 |