How big is this URL you request?

Is there a way to throw a 414 and avoid this behavior when Dancer running under Starman?

Sure, add a check for URI size first in line :) http://search.cpan.org/dist/Dancer/lib/Dancer/Cookbook.pod#Default_route

(untested)

any qr{.{8190}.*} => sub { 
    status '414'; # 414 - Request-URI Too Long
};
[download]

Or make a Plack::Middleware to do the check

(untested)

  package Plack::Middleware::ProhibitRequestUri8190;
  use parent qw(Plack::Middleware);
  use Plack::Util;
  sub call {
      my($self, $env) = @_;
      my $res  = $self->app->($env);
      if( length $res->{REQUEST_URI} > 8190 ){
        Plack::Util::response_cb($res, sub {
            my $res = shift;
            $res->[0] = 414;
            return;
        });
      }
  }
[download]

plack middleware 414 request uri too large