When I'm scripting some process, I usually work it out manually first so I know each step works. So before trying your pgp commands in the script, work out how to decrypt the file just by typing in the command. Once you have that down, putting it into the Perl script is easy.

As mentioned above, the best resource for getting the details for PGP is the PGP documentation.

One last thing; if you consider gpg you may have trouble with decrypting files sent to you by someone using PGP if they use the IDEA encryption algorithm. I think this is the default algorithm for PGP, and it has some licensing requirements. If you can contact the person encrypting the files on the other end, you can ask them to use a different algorithm. For most people, the actual algorithm won't matter and there are fully open source ones available.