I've written my thoughts on the topic in another thread relatively recently. Basically, do play around and roll your own to learn.

When it comes to production code however, you should use an existing module that offers what you need. Modules are under the scrutiny of a larger audience than your own code. Also, using a module reduces the amount of both code and documentation that you have to write as well as maintain. You better have a really, really good reason to sidestep these benefits.

I did write my own CGI GET/POST parser; my own templating system; my own recursive directory tree descent. All of these were as bad as any other newbie's stab at them would be. Some of their deficiencies became obvious to me soon while others I didn't have the experience to think of and had to read about. It was indeed a learning experience I do not regret; but with that experience under my belt, I also wouldn't nowadays even dream of rolling my own for these tasks.

Makeshifts last the longest.