I have two classes: Logger::Local and Logger::Remote.
Each of them implements log method in a different way. One appends message to local file, the other send a message
to remote server. I want these two classes to have the same interface, so I create
abstract superclass Logger that will define such an interface. This superclass also
provides some common methods (for example method instance).
In your scenario call to Logger::Local->instance may return
instance of Logger::Remote class, which is aparently not desired here.
The class returned is not even superclass nor subclass of Logger::Local and the interface could not match.
The fact that Perl is not typed language and does not enforce interface checks like Java does does not mean
that all OO good practice should be abandoned.
Better way is to have such instance method that allways returns instance of
class that it has been called upon. And when some 'class agregation' is desired, it should be done explicitly be overriding this method.