If you are just interested in new .pm files, it's enough to compare the list of files in an old and a new version of a distribution. Usually all files in a distribution are listed in a file called MANIFEST.

However there can be multiple packages per file, searching for those would be more work.

You might also be able to snapshot versions of the modules/02packages.details.txt.gz file found on your CPAN mirror, and compare those versions.