One thing I like to point out when someone plays the "I'm not using a module because I want to really learn Perl" card: a very important part of learning Perl is knowing when to be lazy. Doing everything by hand may help someone learn how things work behind the scenes, but it doesn't usually help learn how to solve real problems in the most effective way. In other words, not only is reinventing certain wheels usually a waste of time, it's counter-productive to the stated goal of "learning Perl".
Using modules is only worth it if you're good enough at [Pp]erl to debug the flaws in someone else's module; this is usually even harder than debugging your own code.
And yes, modules can contain bugs. Heck, even the perl binary occasionally has bugs; the more code you can understand and fix yourself, the less you're held hostage to someone else's failings.
If you have the time to invest in understanding something directly, do it. Using a module is only a sound business decision if you can trust the author to have coded it right; otherwise you're setting yourself up for a pile of extra maintenance and bugfixes. Often, this involves reading and deciphering so much obfuscated module code that you might as well have coded it yourself, in a readible way, in the first place, and saved yourself the mental anguish of dealing with some foreigner's tortured notion of how to write decent English documentation.
In short, if you want the job done right, you get what you pay for. If you're not willing to code it yourself, you have to check it yourself, and that's not better. Ultimately, you're the one responsible for the solutions you provide, so if you can't guarantee them, you can't trust them.
Edited by planetscape - fixed the anonymonk's square brackets which were being linkified