This episode is a holiday episode kind of like Phineas and Ferb Christmas Vacation. Here is the episode.
One day in October at the Flynn + Fletcher house, Phineas and Ferb were invited to a Halloween party by Baljeet. When they got there, Isabella was there, Irving, Buford, Baljeet and the Fireside Girls were at the party. "Hi, Phineas!" Isabella said. Ferb was wearing a builder and Phineas was dressed up as a builder too! Then, Phineas said, "hey where is Perry?" Meanwhile Perry swiped his hat on and went inside a secret passage way under the apple bobbing bucket. Perry zoomed all the way to the Halloween party at his home base. "Hello, Agent P!" Major Monogram said with a vampire costume on. Perry was not so thrilled about Halloween, but he made a smile. Then he saw Carl in a corner talking to a kid. "Hi Agent P!" Carl said to Perry as he walked over. Then Perry noticed who the kid was! It was Irving! Perry swiped his hat off as quick as he can. "No worrys, Agent P," Carl said. "It's just my little brother, Irving," Perry swiped his hat back on. Irving looked at Carl. Is that Perry?" Irving asked. "Yes! Do not tell your friends, Phineas and Ferb that Perry is an agent called Agent P!" Carl said seriously.