In “Phineas and Ferb,” Dr. Heinz Doofenshmirtz knows Perry the Platypus’s name because he has encountered him multiple times during his evil plans. In the show, Perry is often referred to as “Perry the Platypus” or just “Perry,” especially when he’s in his secret agent mode. It’s also a running gag that Doofenshmirtz has a somewhat obsessive awareness of his nemesis, which includes knowing Perry’s name. Plus, in some episodes, he humorously acknowledges Perry’s role in foiling his schemes, reinforcing his familiarity with him.
In “Phineas and Ferb,” Dr. Heinz Doofenshmirtz knows Perry the Platypus’s name because he has encountered him multiple times during his evil plans. In the show, Perry is often referred to as “Perry the Platypus” or just “Perry,” especially when he’s in his secret agent mode. It’s also a running gag that Doofenshmirtz has a somewhat obsessive awareness of his nemesis, which includes knowing Perry’s name. Plus, in some episodes, he humorously acknowledges Perry’s role in foiling his schemes, reinforcing his familiarity with him.