At a minimum, each file should have:
1. A one sentence explanation of what the code in the file does
2. The name of the author
3. A date
4. Some sort of copyright

Some sort of version is also important, but often this comes from an external revision control system.

The most important code comments explain the known defects, limitations and the problems in your code.

Comments tend to say what you hoped your program would do. Just when you need the comment the most, it lies!

When I am tempted to write a comment to explain something, I try to reformulate the comment into an error message. I use something like the or die idiom used when opening files. This gives me nice a spot to put my description of what is going on. It is also easier to maintain. If the this doesn't seem natural, I put in a diagnostic print statement with a trailing if $verbose.

something_tricky() or die "Failed to execute the trick";
print "So far I have $my_intermediate_result" if ($verbose > 0);
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It should work perfectly the first time! - toma