You would be amazed.

My experience is that the process of adding strict.pm to any lengthy script that doesn't have it already goes quickly (because it is largely mechanical), and generally fixes several bugs. (In particular you find that "has been running fine" often means, "Shit, that caused the problem I was having?") It may or may not fix the one I started out looking for, but if it doesn't then I generally come out with a pretty good picture of how the code works, and my development time on that code will be faster.