I won't divulge the name of the company, but a year or two ago, we purchased a license for a software program written in perl to have access to their "source" code (the code they gave you without a license was severely obfuscated, and it was LONG).

What we found was that, in order to avoid having to redistribute modules with their code, they had simply taken the relevant parts they needed from perl modules and simply included them, verbatim into their code. Now, I'm not sure what the licenses say about that, but I'm pretty sure that that particular practice is dead wrong.

As for this specific case, IANAL (ooo...), but I would think that as long as you're not selling the actual interpreter and modules, then you've got nothing to fear. You're allowed to sell perl code. Package the two parts separately, one you pay for (yours) and one you don't (everyone else's).

But just remember, once they have your hands on the perl code, it's kind of hard to stop them from deciphering it, especially if you're using modules which must be called by their names. If they really want to keep managed control of their source, they need to use a compiled language. In my earlier example, we actually did a fair amount of hacking on their code using their munged stuff, simply by tracking which vars went where.