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.