The reorganization took effect last week, according to THR, as a result of conversations between Feige, Horn and Disney CEO Bob Iger earlier this summer.
Perlmutter has a reputation for being controlling and frugal -- a reputation Marvel Studios has earned in spite of the massive scope and budgets of its films. Earlier this year, reports emerged that Perlmutter was unhappy with Marvel's publishing side promoting Fantastic Four and X-Men comics, while parent company Disney didn't own the film rights. The subsequent cancellation of Fantastic Four threw fuel on the fires of those rumors.
Feige is generally credited with building Marvel up into the powerhouse it is in Hollywood. When he came on board, the publisher had recently come out of bankruptcy and licensed away the film rights to a number of its move valuable intellectual properties. He was instrumental in bringing Sony's Spider-Man into the Marvel Cinematic Universe with Captain America: Civil War, and bringing Marvel on board with Sony's upcoming Spider-Man franchise.
Perlmutter remains in his role at Marvel Entertainment and will presumably have indirect input on the films. Being outside of that chain could direct more of his energies in the direction of other parts of Marvel.
It isn't clear what, if any, immediate impact this will have on Marvel's films, although many fans will likely speculate on the possibility that Fox's flailing Fantastic Four franchise could find common ground with Feige where none could be found while answering to Perlmutter.