The perllocal.pod contains a list of locally-installed modules not included with the standard distribution.

If you download and install Perl and then install several CPAN modules, only the CPAN modules will be listed in perllocal.

I belive it's updated during the "make install" process, by code in ExtUtils::MakeMaker.

You can read it with the perldoc command or with the pod2text/pod2html commands.