Although I have used Dancer, I definitely don't consider myself to be an "expert" on Dancer.

Looking through the Dancer documentation, here's one (untested) idea that might work.

use Dancer;
my $max_url_length = 100; # change this to what you want

# first route handler
any ['get', 'post'] => '.*' => sub {
  if ((length request->request_uri) > $max_url_length) {
    # do the 414 error page
  }
  else {pass(...);}
};

# create other routes

dance;
[download]

The basic idea that I was trying to do in the above code was to have the first route handler that will catch everything and check the URL length. If it's above the max limit, give the 414 error page. Otherwise, pass the request on to the next matching route handler. Again, I haven't tested that code, so I can't guarantee that I'm reading documentation correctly.