o   Always check the return codes of system calls.  Good
           error messages should go to STDERR, include which
           program caused the problem, what the failed system
           call and arguments were, and (VERY IMPORTANT) should
           contain the standard system error message for what
           went wrong.  Here's a simple but sufficient example:

               opendir(D, $dir)     or die "can't opendir $dir: $!";