Perl module for comments that do more than just sit there

 Smart::Comments is a Perl module that implements "intelligent" comments that
 can aid in debugging and tracking program execution flow. 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.