The idea behind this module is to provide a sort of plugin architecture for
Perl templating systems. Callbacks are executed by the contents of a request
to the Perl templating server, before the templating system itself executes.
This approach allows you to carry out logical processing of data submitted
from a form, to affect the contents of the request parameters before they're
passed to the templating system for processing, and even to redirect or abort
the request before the templating system handles it.