test.pl

package Local::EmbeddedApp;
use Mojolicious::Lite;

get '/hello' => {text => 'hello from embedded app'};

my $embedded = app;

package Local::HostApp;
use Mojolicious::Lite;

app->routes->route('/embedded')->detour(app => $embedded);

get '/hello' => {text => 'hello from host app'};
get '/:placeholder/firm' => {text => 'placholder'};

my $host = app;

package main;

use Test::More;
use Test::Mojo;

my $t = Test::Mojo->new->app($host);

$t->get_ok('/hello')
  ->status_is(200)
  ->content_like(qr/host/);

$t->get_ok('/embedded/hello')
  ->status_is(200)
  ->content_like(qr/embedded/);

$t->get_ok('/other/firm');

warn Mojo::Util::dumper [keys %{ $host->routes->cache->{cache} }];
warn Mojo::Util::dumper [keys %{ $embedded->routes->cache->{cache} }];

done_testing;