I have collected some "Don't"s in my lightning talk from Copenhagen: http://tinita.de/docs/slides/don_t/slides.vroom.
These are mistakes I made myself or found while refactoring code from others, or when answering perl questions in forums or IRC.
edit: It's a bit short, so my example why one shouldn't use C-style for loops is missing the reason why I decided to include it - I found an off-by-one error. And of course every rule has its exceptions.