Combinatorics is the branch of mathematics studying the enumeration,
combination, and permutation of sets of elements and the mathematical
relations that characterize their properties. As a jumping off point,
refer to:

http://mathworld.wolfram.com/Combinatorics.html

This module provides a pure-perl implementation of nCk, nPk, and n!
(combination, permutation, and factorial, respectively).

WWW: http://search.cpan.org/dist/Math-Combinatorics/