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;
}

[download]

You have an off-by-one error. If @$aref contains 10 elements it will have the indices 0 to 9 and int(rand($#{$aref})) will only pick a number in the range 0 to 8.

Also, perl has a more efficient idiom for swapping values: 

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 ] = @{ $str
+ing[ $i ] }[ $position2, $position1 ];
            }        
        }
    }
    return \@string;
}

[download]

Update: Also you used the wrong comparison operator. Changed ne to !=.

In reply to Re: Problem of referencing into subroutines. by jwkrahn
in thread Problem of referencing into subroutines. by zli034

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