If by decrypt when the user downloads means that the server just spits out a decrypted GPG pipe, and sends plain text over the network to the client, that is fairly easy, but I would'nt call it secure. Anyone can see all your files by sniffing the network.
in reply to Re^2: Encrypt files on server and then decrypt when user downloads
in thread Encrypt files on server and then decrypt when user downloads
So you are stuck sending information thru GPG encrypted mail, because I don't think GPG has a browser plugin to decode web content. Another limited option would be making GPG encrypted files available for HTML download.
You could just implement the old "Caesar's Cipher" for encryption... ;-)
But it isn't Perl, so all I will do, is leave it at that. From what I understand, someone is working on something similar for Perl, call Perlito, but I doubt it will ever eclipse the geniuses at M.I.T who produced Processing and Processing.js.