sub checkISBN {
    my @digits = split(//,uc(shift));
    my $n = scalar(@digits);
    my $sum = 0;
    my $m = 10;
    my $cd;

    if ($n != 10) {
        return (0,($n < 10 ? '-' : '+'));
    }
    else {
        for (0..@digits - 2) {
            $sum += $digits[$_] * $m--;
        }
        $cd = qw(0 X 9 8 7 6 5 4 3 2 1)[$sum % 11];
        return ($cd eq $digits[-1],$cd);
    }
}