Singleton class for mod_perl

 Apache::Singleton is a Perl module that provides a Singleton class suitable
 for use with mod_perl.
 .
 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.
 .
 For a description and discussion of the Singleton class, see
 "Design Patterns", Gamma et al, Addison-Wesley, 1995, ISBN 0-201-63361-2.