From grep:
Note that, because $_ is a reference into the list value, it can be used to modify the elements of the array.
Why would grep work this way? Well, both map and grep are doing the same sort of thing: take a list, do some processing, and put something into a new array. With map, it puts the result from the processing, and for grep it puts the current item if the result is not false. So both probably share the same bits of code to do this.

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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