This is an interesting topic discussed many times in the past; a simple Super Search will show these nodes:

• Plugin based Perl app
• Runtime loading of arbitrary objects, includes code and explanations from davorg and merlyn, introduces the Factory design pattern
• How to a construct objects with variables as class names?, more examples
• Class::Factory -Base class for a factory, lachoy introduces Class::Factory
• using a variable to specify which perl module to use, with an explanation written today by tye :)

Ciao, Valerio