I think better is to emphasize what TO DO rather what not.

At the very top of my list is:
1. Always "use strict;" - this is a compile time directive and has no run time penalty.
2. Almost always run with run time warnings enabled. Turning this off for well debugged apps that are extremely performance critical and written by experienced Perl programmers can be considered. However this should never be your first thought about "how to I speed this up?".