sub _checksum
          {
          my $data = _common_format shift;

          return unless defined $data;

          my @digits = split //, $data;
          my $sum    = 0;

          foreach( reverse 2..10 )
               {
               $sum += $_ * (shift @digits);
               }

          my $checksum = (11 - ($sum % 11))%11;

          $checksum = 'X' if $checksum == 10;

          return $checksum;
          }

Example 3: The Business::ISBN::_checksum routine.