libclass-isa-perl binary package in Ubuntu Precise i386

 Suppose you have a class (like Food::Fish::Fishstick) that is derived, via
 its @ISA, from one or more superclasses (as Food::Fish::Fishstick is from
 Food::Fish, Life::Fungus, and Chemicals), and some of those superclasses may
 themselves each be derived, via its @ISA, from one or more superclasses (as
 above).
 .
 When, then, you call a method in that class ($fishstick->calories), Perl
 first searches there for that method, but if it's not there, it goes
 searching in its superclasses, and so on, in a depth-first (or maybe
 "height-first" is the word) search. In the above example, it'd first look in
 Food::Fish, then Food, then Matter, then Life::Fungus, then Life, then
 Chemicals.
 .
 This library, Class::ISA, provides functions that return that list -- the
 list (in order) of names of classes Perl would search to find a method, with
 no duplicates.

Source package