SYNOPSIS
use Algorithm::CheckDigits;
$isbn = CheckDigits('isbn');
if ($isbn>is_valid('3882291923')) {
# do something
}
$cn = $isbn>complete('388229192');
# $cn = '3882291923'
$cd = $isbn>checkdigit('3882291923');
# $cd = '3'
$bn = $isbn>basenumber('3882291923');
# $bn = '388229192'
DESCRIPTION
ALGORITHM
 1.

The sequence of digits is processed right to left.
Every digit is multiplied with their position in the sequence (i.e.
the digit left to the check digit has the weight 2 then 3 etc.).
With a Hongkong ID (hkid) the leftmost char is replaced with its position in the alphabet and then multiplied with 8 (its weight).
 2.
 The sum of all products is computed.
 3.
 The sum of step 2 is taken modulo 11.
 4.

The checkdigit is the difference of the sum from step 3 to eleven
under the following conditions:

 isbn,issn

If the difference is 10, the check digit is 'X'.
If the difference is 11, the check digit is 0.
 ustid_pt
 If the difference is greater then 9, the check digit is '0'.
 hkid

If the difference is 10, the check digit is 'A'.
If the difference is 11, the check digit is 0.
 wagonnr_br

If the difference is 10, the check digit is 0.
If the difference is 11, the check digit is 1.
 nhs_gb

If the difference is 10, the number would not be taken.
If the difference is 11, the check digit is 0.
 vat_sl
 This is a little bit unclear, don't trust on the method for this type.

METHODS
 is_valid($number)

Returns true only if $number consists solely of numbers and the last digit
is a valid check digit according to the algorithm given above.
Returns false otherwise,
 complete($number)

The check digit for $number is computed and concatenated to the end
of $number.
Returns the complete number with check digit or '' if $number does not consist solely of digits and spaces.
 basenumber($number)

Returns the basenumber of $number if $number has a valid check
digit.
Return '' otherwise.
 checkdigit($number)

Returns the checkdigit of $number if $number has a valid check
digit.
Return '' otherwise.