"But I have to admit something to you. When I first got to L.A., I went to the club. A lot. And I wanted to get into the club, but I had no money, I had no clout, I had nothing, and sometimes I would pose as you to get into the club."
Much to Perry's shock and embarrassment, the New Girl star was already aware because she's a "goody two-shoes" and yet people were always convinced she'd been out partying.
Deschanel replied: "Well, I know this. People were like 'I saw you!' But I'm such a goody two-shoes, and people kept going like, 'I saw you out! I made eye contact with you!'
"Then everybody kept telling me about you, 'This girl Katy, she looks just like you,' and I'm like, 'Who is this Katy?'
"And then when I first met you, I was so relieved because you're so pretty, and I was like, 'Oh, thank God. She's so pretty."
The 36-year-old singer confessed she was always "tuned up to 11" and acted wild when she was in clubs.
She added: "So I'm sorry if I misrepresented you."
But Deschanel - who is kidnapped by aliens who think she is the pop star in Perry's new music video for Not the End of the World - reassured the Firework hitmaker she found the confusion a compliment.