sub mutate{
    my @string = @{$_[0]};
    for my $i (0.. $#string){
        my $aref = $string[$i];
        #print "\n";
        if (rand() < $mutation_rate){
            my $position1 = int(rand($#{$aref}));
            my $position2 = int(rand($#{$aref}));
            if ($position1 ne $position2){
                my $temp = $string[$i][$position1];
                $string[$i][$position1] = $string[$i][$position2];
                $string[$i][$position2] = $temp;
            }        
        }
    }
    return \@string;
}