The most annoying error is that I keep forgetting useful information (like using split with -1 when needed).
What to do?

• Often when I don't recall the precise solution to a problem, I do recall there was something tricky about it, and where or how I found the appropriate documentation (perldoc can be confusing) / pm article.
• I keep notes (on paper!) about my projects.
• I keep code examples that I used successfully, sort of a personal tips and tricks library.
• I also trained my brain to remember things better, e.g. using keywords or even jokes about problems and their solutions. You might find me filing through backup cd's muttering "I know we solved it in 2002 for customer xyz". It works.

There must be other serious mistakes I did, but that was the first that came into my mind ...