In light of Junkward Wars, Perl Style, I offer the following puzzle which limits what you have to work with.

In this case, your task is to write a subroutine that takes in a reference to a list of numbers and returns the average of those numbers, with the limitations that:

• You cannot modify the array elements themselves, or if you do, the array must be restored to it's original state before the subroutine is over.
• You may not declare or use any other variables outside of what perl 'provides' to you. For example, you have to use @_ at some point to get the array, but perl provides this due to the nature of subroutines. You may also use variables like $_ when inside a map/grep/for block, or the use of $1, $2, etc in regex statements when these have been created for you. However, you may not use $_ or any other of the reserved perl variables outside of these types of blocks.
• You cannot use an external module, nor a system command; the calculation should be done entire by perl itself and not call on any other programs.

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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