$ 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

##</code><code>##

use strict;
use warnings;

my @array = qw(A T C G);
my $length = 5;

my @result = add_to_strings($length - 1, @array);
print "@result";


sub add_to_strings {
    my $len = shift;
    my @temp_result = @_;
    return @temp_result if $len <= 0;
    my @temp;
    for my $string (@temp_result) {
        push @temp, $string . $_ for @array;
    }
    add_to_strings ($len - 1, @temp);
}