Solved.

I was able to get the desired behavior by wrapping things in app.pl

#!/usr/bin/env perl
use Dancer;
use REST_App;
#dance;

use Plack::Builder;

my $app = sub {
    my $env = shift;

    if (length ($env->{REQUEST_URI}) > 1000) {
        return [
            '414',
            [ 'Content-Type' => 'text/html' ],
            [ get_414_page() ], # HTML for the error page
        ];
    }

    my $request = Dancer::Request->new(env => $env);
    Dancer->dance($request);
};

builder {
    $app;
};
[download]