|Come for the quick hacks, stay for the epiphanies.
Are they really your modules? I mean ownership, not authorship. Modules written during working hours may very well belong to the company, but there are also companies which adhere to the "all your base are belong to us" point of view, considering as their own even modules you developed in your free time, as long as they are "related to the proceedings of the company".
That aside, ask them how much benefit they have drawn from the countless module authors and perl porters. Tell them that a copyright line mentioning the company will benefit them more than parsimony. Tell them that collaborating is much more productive than avarice. Tell them that modules they release will be tested by cpantesters, and eventually reviewed, bugs will be discovered the developers themselves would never realize, and releasing them will greatly improve their quality - all for free! What would they do without CPAN? How much code would your company have to write? Besides, if you develop a module to scratch some itch, chances are that others have the same itch, and will eventually release a module for that, regardless of your company's policy.
And tell them about backpan... deleting is useless ;-)
update: and tell'em: not the written module, but the understanding and skill which made that module possible, is your and your company's software capital.