A couple more for you (with merlyn's clarification), based on what I've learned over the years:

• Never use programs from Matt's Script Archive? (technically, this goes into the whole 'never run something without understanding what it's doing' category)
• Never assume that a file/socket/db handle/whatever opened successfully
• Never write your own CGI parsing routines without a really good reason to avoid CGI or CGI::Lite
• Never use the single argument form of exec or system
• Never test for taint by looking for known bad characters. (instead, look if there are any characters that aren't known to be good).
• Never pass input to exec or system without first checking for taint.
• Never assume that e-mail happens immediately, or reliably.
• Never trust a Sicilian when death is on the line
• Never waste time optimizing your code before you know where the real bottlenecks are. (see Optimization: Your Worst Enemy and Premature Opmization)
• Never assume that what you're writing is going to be short lived (unless you delete it immediately after writing it).
• Never forget the importance of descriptive, unambiguous variable, function, and package names, or the importance of comments and documentation

I'll probably have more to add later....