When asked that question, Harry admitted...well, he admitted he's been trained to answer this question like an effing pro because he's never going to spill the beans no matter how many times he gets asked.
Check it out around 19:20 in the video, where Harry answers, "Okay, I didn't know that was the speculation." Um...really, Harry? He continued, "Well, because I'm a media trained person, I'd say I think everyone writes from personal experience and I think you draw from lots of different things and songs aren't always necessarily about what people may take them to be about."
That old chestnut. We wouldn't expect Harry to say anything other than this every time he gets asked who "Perfect" is written about. It's tried and true, a solid non-answer answer. Sigh.