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);
}