It has been my experience that, even when i need a rather esoteric functionality, there is usually something at CPAN that approximates what i need. If it does what i need, great, if not, maybe i can modify it, or at least get some ideas on how to code my specific need.
It is quite rare that you need a functional module that no one else has ever concieved of. So, the few minutes it will take to find something on CPAN, download it, install it, and begin learning to use it will surely save you design time to re-invent the wheel.
Also, there have probably been a number of programmers that have at least reviewed the code, and made some tweaks, so it's generally not realistic that you'll come up with a much better solution. It does happen, but you'll save time in the long run by usually taking the existing module.
Perhaps this involves a misunderstanding of why things aren't in the core. The core is not a repository for everything solid and useful, its for base functionality. Just because a module is not included in the core in no way labels it as inferior or inefficient, just not part of the core.