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);
}