$ perl -e 'use strict;
> use warnings;
>
> my @array = qw(A T C G);
> my $length = 5;
> my @result = @array;
>
> add_to_strings($length - 1);
> print "@result";
>
>
> sub add_to_strings {
>     my $len = shift;
>     return if $len <= 0;
>     my @temp;
>     for my $string (@result) {
>         push @temp, $string . $_ for @array;
>     }
>     @result = @temp;
>     add_to_strings ($len - 1);
> }
> '
AAAAA AAAAT AAAAC AAAAG AAATA (... many string omitted for brevity ...) GGGGA GGGGT GGGGC GGGGG