1. Never guess wildly at what may or may not be slow.
2. Don't concern yourself with performance unless it is a problem.
3. When you need performance, profile your code to find out where the problem is.
4. If you really really need a very very fast program then why are you using Perl?

And some quotes from the collection on my home node:

More computing sins are committed in the name of efficiency (without necessarily achieving it) than for any other single reason - including blind stupidity.
— William A. Wulf, A Case Against the GOTO

Rules of Optimization:
Rule 1: Don't do it.
Rule 2 (for experts only): Don't do it yet.
— Michael A. Jackson

Makeshifts last the longest.