Though Perry and Doofenshmirtz regard each other as a nemesis, their relationship is more along the basis of "friendly enemy".
Doofenshmirtz habitually explains his plans before carrying them out and Perry always stops to listen.
Doofenshmirtz always anticipates Perry the Platypus's entrance and even begins to become anxious if he doesn't show up. He also tends to let Perry into his own home or lair should he discover Perry's presence before Perry has the chance to break himself in. In fact, the two of them are so used to one another being in each others lives that they can't be nemeses with anyone else for more than a day or so. They even exchanged genuine Christmas presents. At one point when Perry the Platypus refused to foil his plans, he followed constantly telling him to thwart him.Perry always saves Doofenshmirtz from death, when Dr. D created his own aluminum siding business, Perry rescues him when it is on the brink of explosion.