Smart comments provide an easy way to insert debugging and tracking code into
a program. They can report the value of a variable, track the progress of a
loop, and verify that particular assertions are true.

Best of all, when you're finished debugging, you don't have to remove them.
Simply commenting out the use Smart::Comments line turns them back into
regular comments. Leaving smart comments in your code is smart because if you
needed them once, you'll almost certainly need them again later.

WWW: http://search.cpan.org/dist/Smart-Comments/