Universal is base class for all classes in object oriented perl. universal::isa (isa stands for "is a") is method which can tell you if object is blessed in package or inherits from package.

this module overloads universal::isa with a version that makes sure that, when called as a function on objects which override isa, isa will call the appropriate method on those objects

in all other cases, the real universal::isa gets called directly.