Well, when you write your own code in a language like Perl or Java, you can always try to minimize and localize side-effects. That is, you can try to make most of your functions pure, reduce the need for looping by using higher order functions, doing I/O at the top levels, not willy-nilly in the bowels, etc. When it comes to libraries though, you're at the mercy of someone else. Them's the breaks.