MCCartney joined Perry, Johnny Depp and Alice Cooper in the studio to rework his Badfinger hit Come & Get It and the guitarist admits he couldn't stop smiling as he watched the former Beatles star play.
He tells ABC News, "(Getting to) play it live with the guy who wrote it, and a Beatle was, like, stunning.
"I remember watching them on the Ed Sullivan (Show) when I was 14... I mean, that's why I'm doing what I'm doing today, and to have MCCartney just strolling in like any other cats that played on (the album) was really amazing. I still can't believe it."
And Perry insists Cooper and Depp were just as starstruck: "We were looking at each other like, 'I can't believe this is happening'. We were really grateful to have him come in and cut the track. I mean, for all of us, it was like, 'Holy s**t, we're actually doing a session with Paul McCartney... I mean, talk about an ego leveller."