One optimization I have seen used (though never profiled myself) is to tell perl how big a hash you will need:

my %hash;
keys %hash = 100_000;
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Without this line, perl basically has to build a new hash table several times as the number of keys grows. (Think of it as the hash counterpart of pre-growing an array:

my @array;
$#array = 99_999;
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)

As I said, I've only seen this while reading source code; I don't know how much it really gets you.

the lowliest monk