I'd say always use strict and warnings. Only turn it off for the sections of code where you need to. If you do think that you need to, consider whether there's a better way of doing the job - there almost always is. You generally only need to turn them off when you're expecting something magical to happen or are doing something Just Plain Weird. Magic and Weirdness are hard to read and so hard to maintain. Even with sufficient comments, someone who has to maintain your code later might just have to rip it out and start again if you try to be too clever. And, as well as commenting the input and output of your subroutines, COMMENT YOUR ALGORITHMS. (added shouty bit later)