Thanks, I tried that with this sample code.

get '/TestingAssync' => sub {
  delayed {
    add_header 'X-Foo' => 'Bar';
    flush;
    content 'Hello, world';
    done;
  };
};
And I got this error:

String found where operator expected near "add_header 'X-Foo'"
        (Do you need to predeclare add_header?)

?