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.

WWW: http://search.cpan.org/dist/Params-CallbackRequest/