builder {
    #enable 'RequestId', id_generator => sub {
    #    Data::GUID->new->as_string;
    #};

    enable sub {
        my $app = shift;
        sub {
            my $env = shift;
            $env->{'psgix.request_id'} = Data::GUID->new->as_string;
            my $res = $app->($env);
            # do postprocessing
            return $res;
        };
    };

   ...