I would faint, scream like a little girl and finally i would Kiss her and hug her. I would thank her for being herself and ask her a million Вопросы before she go.
I would stare at her at first and then ask nicely, " Are Ты Katy Perry?!?!" Then she would say yes. Than I would be screaming, "AHHHHHHHH! I Любовь you! OMG! I can't believe it is really you!" Then I would hug and Kiss her and do all kinds of things with her. I Любовь Katy Perry!
i would probably squeal at first, then say hi and tell her how awesome she is and how her Музыка inspires and entertains me, and maybe hug her, if she wouldn't mind :)
I would run up and hug her, tell her how much I Любовь her and ask her if she wanted to be Друзья and swap emails so we could keep in touch! If she was upset and cried in front of me, I'd hold her tight and wouldn't let go till she felt better!