SYNOPSIS
use Algorithm::CheckDigits;
$bwpk = CheckDigits('bwpk_de');
if ($bwpk>is_valid('151058D20711')) {
# do something
}
$cn = $bwpk>complete('151058D2071');
# $cn = '151058D20711'
$cd = $bwpk>checkdigit('151058D20711');
# $cd = '1'
$bn = $bwpk>basenumber('151058D20711');
# $bn = '151058D2071';
DESCRIPTION
ALGORITHM
 1.

Beginning left all digits are weighted 2,3,4,5,6,7,1,6,7,2,3. Letters
are replaced according to the following table:
my %table_to = ( A => 12, B => 14, C => 16, D => 18, E => 20, F => 22, G => 24, H => 26, I => 28, J => 6, K => 8, L => 10, M => 12, N => 14, O => 16, P => 18, Q => 20, R => 22, S => 4, T => 6, U => 8, V => 10, W => 12, X => 14, Y => 16, Z => 18, );
 2.
 The weighted digits are added.
 3.
 The sum from step 2 is taken modulo 11.
 4.

The checksum is 11 minus the sum from step 3.
If the difference is 10, the checkdigit is 0.
If the difference is 11, the checkdigit is 1.
METHODS
 is_valid($number)

Returns true only if $number consists solely of numbers and hyphens
and the two digits in the middle
are valid check digits according to the algorithm given above.
Returns false otherwise,
 complete($number)

The check digit for $number is computed and inserted into the
middle of $number.
Returns the complete number with check digit or '' if $number does not consist solely of digits, hyphens and spaces.
 basenumber($number)

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

Returns the check digits of $number if $number has valid check
digits.
Return '' otherwise.