Hmm seems a slightly prickly issue in an open source forum such as this. If you make your program annonying enough by asking for register/payment constantly or having ads in it then some people may pay. If it is web enabled I'd look at it calling back to your server to check if people have paid or not. As some have said your gonna have a hard time hiding your code.

Storing as much as possible in binary format may help. There is also Acme::Bleach but it always breaks my code. The only way I have thought of doing it is with a C or java binary that stores your key and decrpyts and runs your perl code. Still it could easily be decompiled tho and decrpyting at run time would likely be slow. You could try building in a time limited self destruct feature or just make people register before they download.

If your software is any good tho no matter what you do some guy in Russia with time on his hands is gonna crack it.