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 != $position2 ){
                @{ $string[ $i ] }[ $position1, $position2 ] = @{ $string[ $i ] }[ $position2, $position1 ];
            }		
        }
    }
    return \@string;
}