use warnings;
use strict;

my @array = qw(A T C G);
my $word_length = 4;

my $max = ($#array+1)**$word_length ;
my @count;
$count[$_] = 0 for 1..$word_length;
my $mainc = 0;

for my $mainc (1..$max){ 
    print $array[$count[$_]] for 1..$word_length;  
    $count[1]++;

    for my $counter (1..$word_length) {
        if ($count[$counter] == $#array){
            $count[$counter] = 0;
            $count[$counter + 1]++;
        }
    }

    print "\n";
    $mainc++;
}