The Championship leaders took a second-half lead at White Hart Lane through Jorge Grant, before goals from Ryan Mason, Roberto Soldado and Harry Kane in the final 20 minutes turned the game on its head.
"There was a lot of young players on the pitch for us tonight and I thought they did outstandingly well, as well as the senior ones that hadn't played a lot of football," Pearce is quoted as saying by ESPN.
"We put a team out that we felt was going to win the game and I think Tottenham will be more than pleased that they managed to get through in this competition and not get beaten at home again. They had a big scare.
"From my point of view, I am very proud of them. I am very disappointed we lost the game against a very good side that cost multi-millions to assemble. A few of our players have given our players a big lift. The fans echoed that tonight, the Forest fans, that made an atmosphere in this game."
Spurs will now host Brighton & Hove Albion in the fourth round of the competition.