Given the amount of time I hang-out in book space, I would have liked to see something along these lines for ISBN numbers. When last I checked CPAN there was at least one solution (more actually) but most required something along the lines of use Kitchen::Sink; so after a bit of looking around I came up with:

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);
    }
}
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Don't know if this is what you had in mind, but I found it useful...

--hsm

"Never try to teach a pig to sing...it wastes your time and it annoys the pig."