I love Smart::Comments! Basically, it requires you to write debug comments like this:

sub foo {
   ### entering foo
   my $x = shift;

   ### x: $x
   return $x + 1;
}

### starting...
my $y = foo(42);

### y: $y

### finish
exit;
[download]

As you can see, they're just simple comments, until you use Smart::Comments, when they'll magically print the following output to STDERR:

### starting...

### entering foo

### x: 42

### y: 43

### finish
[download]

The best part is that, when you disable Smart::Comments, they will sit quietly there -- just like comments they are -- documenting your code, but not being executed.

Simple, useful... smart!!! :)