User blog comment:Black Spiderman/Phineas and Ferb: Across The Second Dimension/@comment-753210-20100311011145

But isn't his nationality American. And he's with Phineas and Ferb for a long time. Good idea though.

I have some guesses here. Phineas and Ferb doesn't really understand Perry, right? So he uses the headphone version of their translator. Then for the episodes after the movie, there are two possibilities.

1. They'll forget that Perry is an agent via memory eraser or they don''t believe that he's an agent since they've only met him at the second dimension.

2. The "Where's Perry" running gag will change a bit, since they do know that Perry was an agent at those episodes.