If you've ever worked with Perl's various Tie::* base classes, the idea is similar: you start out with a base class with callbacks defined that don't do anything very exciting, but by their existence satisfy all the subroutine calls triggered by SAX events. In your own driver class, you simply redefine the subroutines that should do something special and let the default behavior rule for all the events you don't care much about.

The default behavior, in this case, gives you something nice, too: access to an array of strings (stored as an instance variable on the handler object) holding the XML document that the incoming SAX events built. This isn't necessarily very interesting if your data source was XML, but if you use a PerlSAXish driver to generate an event stream out of an unsuspecting data source, then this feature is lovely. It gives you an easy way to, for instance, convert a non-XML file into its XML equivalent and save it to disk.

The trade-off is that you must remember to invoke $self->SUPER::[methodname] with all your own event handler methods. Otherwise, your class may forget its roots and fail to add things to that internal strings array in its youthful naïveté, and thus leave embarrassing holes in the generated XML document.

5.5. A Handler Base Class		5.7. XML::SAX: The Second Generation