I am a huge fan of Dancer and Dancer::Plugin::DataFu. It doesn't provide the client-side javascript you are looking for out of the box, but you could extend it to do so by customizing the templates that ship with the module.

Since DataFu builds it's server-side validation from the form profile, you already have the hooks you need to generate some javascript validation and insert it into the form.tt template.

This would be a great addition to the module that you could send to the module developer and help the Dancer community as well.