Loose forward Victor Vito, another experienced All Black, was also omitted from the 28-man party but captain Richie McCaw was recalled after a sabbatical for the championship, which begins with New Zealand's visit to Australia on August 17.
Hansen did not close the door on Weepu, who earned his 71st cap against France in June, but said his inability to deal with the pace of the modern game was preventing him from exploiting his considerable skills.
"We just felt that the way we're trying to play the game, the speed we're trying to play the game, Piri was struggling with that," Hansen said in a teleconference from Christchurch.
"His last game really confirmed what our thoughts were. We just think it's time to say, 'you go away and see if you can sort out that issue' and meanwhile young TJ can have a crack.
"Perenara has got a similar skill set - a younger version, a fitter version. We just think it's time."
Weepu, 29, has been in and out of the All Blacks team since his international debut in 2004 and indicated he would not be giving up on his test career, tweeting: "Down but not out.. As always".
Aaron Smith and Tawera Kerr-Barlow are the other halfbacks in the squad with the former likely to get the starting berth on the back of his 15-test experience and more accurate passing game.
Vito won his 22nd cap in the 3-0 series sweep over France in June but had paid the price for not being intimidating enough, although the door was not closed on him either, Hansen said.
"I think we all know Victor is a great athlete but the thing we've been asking Vic to do is go out and play with a sense of freedom and real aggression and physicality," he said of the 26-year-old number eight.
"It just hasn't come. We would like him to step out of the pressures of All Black rugby and try and see if he can develop those things at a lower level."
Rene Ranger was unavailable for "family reasons", the New Zealand Rugby Union said, even though Hansen said he would consider the France-bound back after he impressed in the June internationals.
With such a small squad, Ranger's ability to play across the backline would be missed, Hansen said.
"Having a guy like that on our team was important, particularly a guy who could cover the midfield and wing," he said.
"The fact that he's not there forces us to have a rethink and definitely made us want to have another specialist winger in the 31."
The "31" includes lock Jeremy Thrush as well as Auckland Blues backs Francis Saili and Frank Halai, who will all train with the squad but return to their provinces on the Wednesday of each test week.
Canterbury Crusaders prop Joe Moody was named as injury cover for Wyatt Crockett, who was included in the squad but will miss the start of New Zealand's title defence with a knee injury.
Squad:
Forwards - Dane Coles, Andrew Hore, Keven Mealamu, Wyatt Crockett, Charlie Faumuina, Ben Franks, Owen Franks, Tony Woodcock, Brodie Retallick, Luke Romano, Samuel Whitelock, Sam Cane, Steven Luatua, Richie McCaw (captain) Liam Messam, Kieran Read.
Backs - Tawera Kerr-Barlow, TJ Perenara, Aaron Smith, Beauden Barrett, Dan Carter, Aaron Cruden, Ma'a Nonu, Conrad Smith, Israel Dagg, Charles Piutau, Julian Savea, Ben Smith.
(Reporting by Nick Mulvenney in Sydney, editing by Ian Ransom)
