I've narrowed it down to this simple use case:
get '/Load/:ID' => sub {
    my $ID = params->{'ID'};
   delayed {
      my $TextToWrite = ReturnText(); # Just returns "hi", works fine.
      my $Username = "Test";
      session 'Username' => $Username;   # Causes the whole thing to to return "400 Bad Response" and not work at all.
      header 'Content-Type' => "text/html; charset=UTF-8";
      flush;
      content "$TextToWrite";
      done;


   };
};

Notice the "session 'Username' => $Username."

That causes a "400 Bad Response" error. I'm guessing something to do with the session being set and returning before other stuff has happened??

Thanks,

David