Lots of good reasons, some of which you point out, and some of which you neglect; here are a couple of the ones you neglect...

• Learning. Sure, Perl is a language for getting your job done, but there's no reason it can't also be a language for learning. Sometimes your entire purpose in writing something is to learn, and sometimes you don't just want to learn how to make it work; sometimes you want to understand the internal details of how it works. Sure, you could study an existing module, but sometimes writing the code yourself gives a clearer understanding. This is the same kind of reasoning that leads people to read RFC documents instead of (or in addition to) tutorials.
• Sometimes the existing module, despite technically doing the same thing you want to do, isn't right for your needs. Maybe its interface is needlessly complex for a simple task; maybe the module on CPAN uses XS, and you want a solution you can use easily on different systems without compiling anything. Maybe you need to be able to distribute it as part of something and don't feel like hiring lawyers to figure out whether the license of the module on CPAN allows this. (I'm a programmer, Jim, not a lawyer.) Maybe a lot of things.

In short, my criterion for rolling my own is this: I'm aware that I'm re-inventing a wheel, and I have a good reason for doing it. Do I see the value in avoiding reimplentation by using existing solutions? Yes, of course; I can't count the number of CPAN modules I've used in the last year. But sometimes, just sometimes, there's a reason for doing otherwise.

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