Building the hash programmatically would be ani nteresting brain teaser.

Here is the worst way to do it:

my @strings = (grep /[acgmt]{2}/, ('aa' .. 'tt'), 
               grep /[acgmt]{3}/, ('aaa' .. 'ttt'), 
               grep /[acgmt]{4}/, ('aaaa' .. 'tttt'));

my %sort_cache;

for my $key (@strings) {
    $sort_cache{$key} = join '',sort split('',$key);
}
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Hey, don't take this seriously ;-) it does the job but it's so inefficient it's scary.
Guillaume