in reply to Hash of Hashes

A Hash of Hashes will be in this form:

$hasher{$key}{'population'} = $population; $hasher{$key}{'g_marker'} = $genetic_marker;

You won't be able to use that fast hash assignment as you have written to populate it, but it will be much more flexible in the range of data it can contain. You can still do everything else.