As you describe it, the (memory) bottle neck is when you
start processing the array. Shifting off the elements at
each iteration will save memory - but the memory isn't
released back to the OS - it just means that it takes longer
before Perl needs to ask more memory from the OS. So, wether
it helps to speed up the program depends on what else you do
in the program - if you won't use much more memory, there's
little gain. OTOH, shift is a fast operation, so there's not
much reason to not do it.
If however you are looking for a real performance boost,
look into an algorithm that doesn't require the entire array
to exist at one time. The fact you don't need an element anymore
after you process it suggests you may not need all the elements
before processing the first one either.
Abigail