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 questions before she go.
I would stare at her at first and then ask nicely, " Are you Katy Perry?!?!" Then she would say yes. Than I would be screaming, "AHHHHHHHH! I love 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 love Katy Perry!
i would probably squeal at first, then say hi and tell her how awesome she is and how her music 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 love her and ask her if she wanted to be friends 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!