This module calculates the Modulus 10 Double Add Double checksum, also known
as the LUHN Formula. This algorithm is used to verify credit card numbers and
Standard & Poor's security identifiers such as CUSIP and CSIN.

You can find plenty of information about the algorithm by searching the web
for "modulus 10 double add double".

WWW: http://search.cpan.org/dist/Algorithm-LUHN/