They are very useful in debugging programs as they will tell you the current file that is being interpreted (eg the perl file), and what line number you are currently at. If you have a custom die message for a module, you can mention "died at line " . __LINE__ . " in " . __FILE__ in order to convey as much info to the programmer.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain