The Class:Multimethod module exports a subroutine (&multimethod) that can 
be used to declare other subroutines that are dispatched using a algorithm 
different from the normal Perl subroutine or method dispatch mechanism.

Normal Perl subroutines are dispatched by finding the appropriately-named 
subroutine in the current (or specified) package and calling that. Normal 
Perl methods are dispatched by attempting to find the appropriately-named 
subroutine in the package into which the invoking object is blessed or, 
failing that, recursively searching for it in the packages listed in the 
appropriate @ISA arrays.

Class::Multimethods multimethods are dispatched quite differently. The 
dispatch mechanism looks at the classes or types of each argument to the 
multimethod (by calling ref on each) and determines the "closest" matching 
variant of the multimethod, according to the argument types specified in 
the variants' definitions (see "Finding the "nearest" multimethod" for a 
definition of "closest").

WWW: http://search.cpan.org/dist/Class-Multimethods/