Agent P's theme super spy theme song, which the background scats are featured in pretty much all episodes.
Backing scat: (Dooby dooby doo-ba, dooby dooby doo-ba, dooby dooby doo-ba, dooby dooby doo-ba)
Female Singer: Perry!
Male Singer: He's a semi-aquatic, egg-laying mammal of action
Backing scat: (Dooby dooby doo-ba, dooby dooby doo-ba)
Male Singer: He's a furry little flatfoot, who'll never flinch from a fray!
He's got more than just mad skill
Female Singer: (Wah-ah-ah)
Male Singer: He's got a beaver tail and a bill
Female Singer: (ah-ah)
Male Singer: And the women swoon whenever they hear him say:
Perry: (chatters)
(Fainting sighs from women)Male Singer: He's Perry, Perry the Platypus!
Major Monogram: But you can call him Agent P.
Male Singer: Perry-
Major Monogram: I said you can call him Agent P.
All Singers: Agent P!
~~~~
Also, when Candace was placed in Perry's body, and vice versa, for an episode... the theme had to be changed as well...
Female Singer: She's a semi-neurotic, teenage girl of action
Backing scat: (Dooby dooby doo-ba, dooby dooby doo-ba)
Female Singer: She's a feisty little redhead with a platypus' brain
You best leave her alone,
She's playing MP3s on her phone
And whenever she's around, you can hear the bad guys say:
Dr. Doofenshmirtz: Great, now I have this song stuck in my head!
Female Singer: She's Perry, Perry the teenage girl!