perl allows you to create objects, and then you can define methods which those objects can call. XML::XPath is just some programmer's module that you can use to create XML::Xpath objects and call methods on those objects. exists() is one of those methods. It says so in the documentation.

But just because one perl programmer creates a module that defines a method does not mean that all perl objects can call that method. Objects can only call methods that were specifically defined for that object(or that they inherit from parent objects). XML::Node objects and XML::XPath objects are not the same thing.

Read ikegami's post again.