Custom constraints or custom validators are now possible via FormValidator::Simple::Plugin::CustomConstraint. Read my review of the FormValidator::Simple for more information.
Simple and functional form validation module ( as the name implies ). It does form validation and nothing more. It does not try to generate html for your forms or see the future. There are other modules for this. One thing that it lacked was custom validators. Today I pushed FormValidator::Simple::Plugin::CustomConstraint which should address that issue and add support for custom validation rules.
Since Data::FormValidator and Catalyst::Plugin::FormValidator are no longer recommend for use by Catalyst apps ( as on the Catalyst::Plugin::FormValidator ), this module seems like a viable choice for those who only want form input validation. There is also FormValidator::Lite which is a relatively new addition.