Lawrence Fletcher's line, "Well, this isn't much of a horror movie. Where are the rock and roll musical numbers?" near the beginning of the "The Curse of Candace" segment is a reference to voice actor Richard O'Brien's work in Rocky Horror Picture Show (actor; screenplay author).
The werewolf is voiced by Michael J Fox, who also played a werewolf in the movie Teen Wolf.
The movie Stacy and Candace are watching is not only a clear reference to the "Twilight" book and movie franchise, all three of the characters are voiced by actors who have played vampires, werewolves, and otherwise. Michael J. Fox played Scott Howard in Teen Wolf (1985) where he turned into and werewolf and played the werewolf. Stephen Moyer and Anna Paquin played the vampire and "Kristen" respectfully and they played the vampire Bill and Sookie Stackhouse in True Blood (2008). The name "Kristen" is also likely a reference to Kristen Stewart who played the main character in the the Twilight movies.
When Doofenshmirtz turns into a cow, he also changes gender from male to female.
The movie posters at the Megaplex all spoof movies released in 2008. "Early Evening" (Twilight), "Leap" (Jumper), and "New Zealand" (Australia).