This is the Class::Singleton module. A Singleton describes an
object class that can have only one instance in any system. An
example of a Singleton might be a print spooler or system
registry. This module implements a Singleton class from which
other classes can be derived. By itself, the Class::Singleton
module does very little other than manage the instantiation of a
single object. In deriving a class from Class::Singleton, your
module will inherit the Singleton instantiation method and can
implement whatever specific functionality is required.

WWW: http://search.cpan.org/dist/Class-Singleton/