As a further expansion: The difference between _Print_Me($self) and $self->_Print_Me is that _Print_Me($self) will always use the current package's _Print_Me, while $self->_Print_Me will find the appropriate _Print_Me for the package (class) that $self is blessed into.

As a general rule, you'll want to use _Print_Me($self) when you want to use one specific implementation, no matter what, and use $self->_Print_Me when you want to allow for polymorphism (which should be almost always, since you never know when $self might belong to a subclass which has overridden _Print_Me).