I have been using Data::FormValidator, but recently Validate::Tiny has
caught my attention and it seems to match Dancer's minimalist
philosophy. Someone recently posted a plugin to the Dancer mailing
list: https://github.com/kalgan/Dancer-Plugin-ValidateTiny

Also, you can find it on CPAN: http://search.cpan.org/~kalgan/Dancer-Plugin-ValidateTiny-0.04/lib/Dancer/Plugin/ValidateTiny.pm
Well documented on version 0.04. Hope, you like it.