Polymorphism

As you can see, Perl has only a loose definition of type with respect to objects.  This simple, naive approach allows objects of different classes to be used the same way without any special ceremony.

If two objects have a foo method that means the same thing, Perl will just call it without caring about the type.

This is commonly called "duck typing," and prevents Perl programmers from crafting elaborate type hierarchies to accomplish simple polymorphism.