Following her well-received guest turn on How I Met Your Mother last year, Katy Perry is returning to the world of sitcoms for another go-round.  Katy will guest-star on the March 6 episode of the Fox comedy Raising Hope, playing a prison attendant named Rikki who claims to be a friend of Sabrina's -- played by Shannon Woodward -- from back in the day. How did Katy hook this up?  In a statement, she said, "Shannon Woodward is my best friend.  A couple weeks ago, I was home, just toasting up a frozen chicken fillet when she called me up and asked if I wanted to do something ‘fun’ for a change and try out a guest role on her TV show.  I thought, ‘Sure, I have nothing else to do, and I like chillin’ with my best friend.’"  No wonder Katy and Shannon exchanged tweets earlier this week alluding to something big they had to tell fans. The show's executive producer, Greg Garcia, said in a statement, "She came in and did a terrific job.  She was hilarious.  And now people are telling me that Katy is also a singer.  I sincerely hope that her appearance on the show helps her get her singing career off the ground.  It's a tough business.” In the episode, Sabrina gets arrested after taking part in an Occupy protest, and gets sentenced to two weeks in prison.  Rikki, played by Katy, is determined to make her stay behind bars as horrible as possible -- until she remembers they were childhood friends.