"Back in Gimmelshtump", sung by Heinz Doofenshmirtz, takes place after said character explained his scheme to Perry the Platypus in "Rollercoaster: The Musical!" of Phineas and Ferb.
Dr. Doofenshmirtz: I know you're probably expecting some tragic backstory to explain why I want to reverse the rotation of the Earth. But I'm not gonna go there, ya know, I...well, I'll tell ya!
Back in Gimmelshtump,
I always had to keep it real!
'Cause when you grow up in Gimmelshtump,
Young Heinz: Your childhood is less than ideal!
Dr. Doofenshmirtz: So, I don't dwell on how I crawled my way out of the gutter!
No, I don't!
When people ask me what I mean, I say,
"D-D-D-Do I stutter?"
But I'm no longer a hater,
'Cause I got an Inator
And an over-complicated scheme.
I get a couple of checks
Every month from my ex,
So I can finance my coming regime!
And though my past was a wreck,
I won't let it affect
My aspirations to cause mass hysteria,
'Cause I'm as sure as can be
That it's my destiny
To be the ruler of the Tri-State Area!