But see, I consder Perl modules to be extensions of the language environment itself.

Fine. I agree with you. But guess what? The manager who objects probably doesn't, or s/he'd allow you to use the module, yes? You'll be more persuasive if you set aside your own viewpoint for a moment and try to address the manager's concerns in ways that make sense to the manager.

For (too) many organizations, it doesn't matter that Perl's licensing is flexible and that modules are easy to install. Using modules save programmers lots of time, but represent additional work for someone, and that costs $$$. Your argument will be more effective if you take the time to find out (or estimate) that cost and then show how much more money is saved by using the module anyway (a cost-benefit analysis). Trying to argue that the manager is looking at it "incorrectly," or that there aren't any associated costs, is just more likely to make him/her stop listening.