$req->respond ({ content => ['text/plain', sub {
   my ($data_cb) = @_;
   # 200 / text/plain already sent to client by now...
 
   # do async io that eventually calls the call back...
   myPromise()->then(sub{
      $data_cb->("OK"); # Too late to change content-type or status code here..
      $data_cb->();
   })->catch(sub{
      $data_cb->("Failed"); # Or here...
      $data_cb->();
   });
 
} });