sub check_abn {
    my ($number) = @_;
    $number =~ tr/ //d;
    return if 11 != length $number;

    my $sum = 0;
    $sum += (2 * $_ - 1) * substr $number, $_, 1 for 1 .. 10;
    $sum += 10 * (substr($number, 0, 1) - 1);
    return $sum % 89 == 0
}