Where do you define and fill $res?

That variable is never defined or filled and therefore is undefined.

Maybe you should consider to use strict and warnings.

I had another look at your script and think, there are more flaws than the one I previously mentioned.

• $req should contain a HTTP::Request object; so you should create one.
• POST 'http://...' is missing the correct operator between POST and the URL string
• $ua->request() returns a response object; you should give the variable a better fitting name than $request.