Speeding it up is only possible by using less memory at a time.
Not if what he is trying to do is has a complexity greater than O(n), i.e., if he is entering into another loop of some kind for each element (or even for just every ith element).
Well, yeah, he could do anything else in his program that could be made faster. But since we don't know the rest of his program, nor is his question about that, it's nothing more than pure speculation, and rather pointless.
Perhaps he's recompiling perl each time he inserts a hash element.