As I didn't trust my own rolling of a Perl Singleton I grabbed that code directly from from CPAN's Singleton.pm. Though I'm guessing that the use of the Trinary in the CPAN version might give a performance boost over the clearer functionality your code expresses.

Isn't that why it's called a singleton?

coreolyn

Consider this:

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.