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

##</code><code>##

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