implementation of a "Singleton" class

 The Class::Singleton module implements a Singleton class from which other
 classes can be derived. 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.
 .
 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.
 .
 For a description and discussion of the Singleton class, see
 "Design Patterns", Gamma et al, Addison-Wesley, 1995, ISBN 0-201-63361-2.