Lets your class/object say it works like something else

Perl 5 doesn't natively support Java-style interfaces, and it doesn't support Perl 6 style roles either. You can get both of these things in half a dozen different ways via various CPAN modules, but they usually require that you buy into "their way" of implementing your code. This package overrides the isa() method, allowing your class to claim it's a class it's not (that is, isn't in @ISA).