Why wait for Perl 6? :-)

use strict;
use Language::Functional ':all';

sub match { $_[0] =~ m/^[acgmt]*$/ }

my $x = filter( \&match, ['aa'..'tt', 'aaa'..'ttt','aaaa'..'tttt'] );
print show $x;
[download]

Update - not quite like it, however, perl will still generate that list, but I'm looking at L::F and there's enough there that you can generate the lazy list for processing via double Map's.

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
It's not what you know, but knowing how to find it if you don't know that's important