In addition to Joost's comment, I'd like to point out that pgp will look at the available secret keys on the keyring and tell you which one (if any) should be used to decrypt. Also, I'm not clear if the encrypted filename can be inserted in the middle like that. I would use something more like the following:

I'm not sure how far along your perl skills are, but two modules on CPAN that you might be interested in (if you know how to use them) are Crypt::OpenPGP and Crypt::GPG. Crypt::GPG uses the gpg command to do its work, while Crypt::OpenPGP handles all the work of pgp/gpg on its own. There may be one that's made to work with the free PGP that you're using, too.

Update: Check out Crypt::PGPSimple, too.