Katy Perry and John Mayer attend the 55th Annual GRAMMY Awards Pre-GRAMMY Gala held at The Beverly Hilton on February 9, 2013 in Los Angeles, California. Katy Perry and John Mayer have once again said goodbye -- for now. The pop songstress, 28, and singer-songwriter, 35, have split for the second time, multiple sources confirm to Us Weekly. The high-profile pair began casually dating during summer 2012, and briefly called it quits back in August before reconciling the next month. "It's sad," one pal tells Us, but cautions that it might not be a permanent break. "It's not over until it's over. You have to see how things play out." Adds another source: "She's leaving the window open. They have both been so focused on work." Indeed, Perry has been busy recording her third studio album in Los Angeles, while Mayer -- on the mend following serious vocal problems and throat surgery last year -- is prepping for upcoming tour dates an