There is really no easy way to do it in perl. There are workarounds, but they are probably more trouble than they are worth. I assume these are scripts you are sending to other people? Two points:

• 1) Use a C executable to stop "source-code" theft. Even that can be somewhat decoded.
• 2) Why do you think a copyright would not work? If it took months and years, as you say, then it should be very, very unique and easily identifiable as yours. Sure, someone may steal it and use it, but if you find someone (e.g. a business competitor) doing so, you can sue them and get them to stop.

Frankly, you are not going to find a lot of support in the Perl community, as perl has a very open (and open-source) philosophy than encourages sharing your source code, learning from others, and a deep dislike for all things secret and propreitary.

As a final note, you could always at least make it *difficult* for people to steal and modify your code by obfuscating it. See the Obfuscated Code section for some bad (or good, depending on how you look at it) examples.