my @parents = @population[40 .. 49];
    shuffle @parents;

    my @children;
    push @children, crossover( pop @parents, pop @parents )
        while @parents;

    @population[0 .. 4] = map { mutate($_) } @children;

##</code><code>##

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;
}

##</code><code>##

                my $temp = $string[$i][$position1];
                $string[$i][$position1] = $string[$i][$position2];
                $string[$i][$position2] = $temp;