Perry and Ferb's relationship

Since Ferb doesn't talk very often, his relationship with Perry is shown more through actions than what he says.Ferb seems to share close to the same relationship as Phineas does with Perry.

Perry's Impression of Ferb
Perry is very protective of his owners and will take immediate action if they are threatened ("Journey to the Center of Candace"). He was very upset when he was temporarily reassigned to a new nemesis and looked like he wouldn't get to live with Phineas and Ferb anymore ("Oh, There You Are, Perry"). He was also quite distressed when he was almost relocated because his secret identity was uncovered.

Perry had a nightmare about being relocated once, and woke up quite anxious. He was noticeably relieved when he found out that it was only a dream ("Phineas and Ferb Get Busted!").

Perry has a locket with pictures of Phineas, Ferb, and himself in it. It was given to him by his owners. He apparently modified it to make it into a key that will allow Phineas and Ferb to help him in an emergency ("Phineas and Ferb The Movie: Across the Second Dimension").

Ferb's Impression of Perry
Ferb cares a lot about Perry because Perry is his pet. The boys adopted him because he looked at both of them at the same time, due to his blank "pet mode" expression. They have been together for five years. Ferb's picture was put in a locket that the boys gave Perry when they got him. It is possible that Ferb is more aware of Perry's double life as a secret agent than Phineas is, but it has not been confirmed by him ("Phineas and Ferb The Movie: Across the Second Dimension").

Ferb was worried about Perry when he missed breakfast one day and spent the entire day helping Phineas try to find him. He sang in Come Home Perry to summon Perry home, giving him a hug when he finally showed up ("Oh, There You Are, Perry").

When they traveled to the 2nd Dimension, Ferb didn't seem as shocked as Phineas that Perry was a secret agent, and even defended him, although Ferb tends not to show much emotion even when he feels it. He and his second-dimension counterpart did, however, shed a tear when Phineas-2 hugged Perry, obviously touched. Phineas is much more upset by comparison, but Ferb doesn't stop him from sending Perry away when he tries to sneak off again. He helped on the failed rescue mission to save Perry after Perry had turned himself in. When Perry was about to be relocated, Ferb mentioned that erasing everyone's memories of the day was the only logical option, as "We've had a lot of great days, but we only have one Perry." They share a hug before firing up the "Amnesia-inator" ("Phineas and Ferb The Movie: Across the 2nd Dimension").

Ferb also defends Phineas when he claims to have seen Perry in Africa. He again is happy to see Perry when they find him ("Where's Perry? (Part Two)")