"Doctor! Doctor! I just broke my arm in 17 places!"
"Well, for goodness sakes don't GO in those places, then!"
(thereby finishing the introductory joke).
I've been writing some code for a client's application that deals with sensitive data -- sensitive enough that I wanted to avoid printing out the data EXCEPT during testing. Looking through CPAN, I thought "Aha! Devel::StealthDebug looks like just the ticket! I can print out the sensitive data during testing, then I can comment-out the code in production so there won't even be a trace of the print statement."
Looking back, I'd just as soon start dereferencing NULL pointers in C as do what I did. Turns out that Devel::Cover will silently refuse to output any coverage data if you modify your code after you have started testing. (I had used a kludge of writing fake tests to turn on and turn off the load of Devel::StealthDebug by modifying the code of the module I was testing.)
Why, you might ask (or you might not -- I don't know you) didn't I continue to use Devel::StealthDebug? Because it is currently implemented as a source filter, which (I think) means that it has to be loaded by the module that is directly using it in order to work. Moving the load of Devel::StealthDebug to a separate module means that the separate module can use Devel::StealthDebug's printing facilities, but the main module cannot use Devel::StealthDebug's facilities anymore.
(Just as a point of interest, I have been programming in Perl for 12 years now but this is my first self-modifying code.)