Dr. Doofenshmirtz from Phineas and Ferb obviously knows Perry's name; it's even in his catchphrase ("Curse you, Perry the Platypus!"). But Perry doesn't speak or anything.
How does Dr. Doofenshmirtz know Perry's name?
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From the Phineas & Ferb wiki comes the following quote by Doofenschmirtz from the season 4 episode Chears for Fears:
Dr. Doofenshmirtz: Alright, so what are you afraid of, where is it?
What the hay? Nothing?
(A business card slips through the ropes. Doof takes the card and reads it.)
"Perry the Platypus: O.W.C.A. agent - Fearless." Huh. That figures. I guess you'll just hafta...hang around! Ha ha ha! Oh, wait, this one's better: I'll just leave you here because you're all tied up! Ha ha ha! W-W-Wait, one last one, you're gonna love it: See ya later because it isn't my tail that's upside down!
No? No no, I shoulda stuck with the first two, you're right.
So it is clear that Perry (and presumably every O.W.C.A agent) carries a business card (just as @AnkitSharma suggested in his comment) and we can also assume that once upon an earlier meeting Perry showed that card to Doofenshmirtz.
A second possibility from a theoretical point of view. The show hinges so much on surreality and breaking standard animation conventions. For example most of the music on the show has been shown to be diegetic. (Intentional and of origin within the story). Example: Doofenshmirtz' theme being recorded by him.
As a result it is possible that Perry's theme is heard by Doof anytime Perry shows up.
This is just a thrown out theory because to be honest... it's P&F where the answer "Doof, are you one of those people who's always just known Perry's name" would be "Yes, Yes I am."