Mr. Muskrat has already answered your question directly, but I hope I can provide some useful ideas:
If your users are technically knowledgeable, you might find they replace the older files with newer files to avoid your 30 day restriction. In this case, you might want to compare the contents of all files within each user's home directory with the contents of all files that were previously there.
You could use Digest::SHA1 or Digest::MD5 as a checksum for the contents of each file, perhaps stored in a DBM database or similar. File::Find::Rule::Digest might help.