Cuse and Lindelof made the surprising casting announcement at the close of a Lost presentation at the winter TV previews.
"So finally your Libby questions will be answered," said Cuse, referring to the character's enigmatic — and incomplete — backstory. "No, they will not," joked Lindelof.
Perrineau, who played put-upon single dad Michael Dawson, left the cast at the end of Season 2 after Michael shot and killed Ana Lucia (Michelle Rodriguez) and Libby (Watros) before leaving the island. Michael returned briefly in Season 4 long enough to redeem himself, with the help of a spectral Libby. Michael is believed to have died in the freighter explosion.
Perrineau and Watros are the latest in the list of believed-dead characters who are scheduled to reappear in Season 6, including Elizabeth Mitchell's Juliet, Ian Somerhalder's Boone, Dominic Monaghan's Charlie and Katey Sagal's Helen.
Lost's final season premieres on Tuesday, Feb. 2 at 9/8c on ABC.