Params::CallbackRequest provides functional and object-oriented callbacks
to method and function parameters. Callbacks may be either code references
provided to the new() constructor, or methods defined in subclasses of
Params::Callback. Callbacks are triggered either for every call to the
Params::CallbackRequest execute() method, or by specially named keys in
the parameters to execute().

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.