The Saints grabbed a hatful of goals in a comfortable 5-1 defeat of the Championship's bottom side Barnsley after making nine changes to a team held to a 1-1 draw by Sunderland on Saturday.
Boss Mauricio Pochettino kept his £36million worth of summer recruits in Daniel Osvaldo, Victor Wanyama and Dejan Lovren on the south coast, instead turning to some of his fringe players who did not disappoint.
Perez said: "We are delighted by the performances of the lads and we're very happy.
"In the first half it was a little difficult for us to play because Barnsley played with passion, discipline, and tactically.
"But in the second half we played well, created a lot of chances, and the players from the bench made a good impact, so it was a very good game from us.
"We know we have a good squad. Although you can only play 11 in any game, the others are working well in all positions, and the young players are doing well.
"We've a good project going with the academy, and you can see that with the opportunities the manager has provided for them."
It was expected £15million record buy Osvaldo would be given his first start since his recent switch from Roma.
Perez, however, conceded Pochettino decided against it, adding: "He trained hard this morning as we are preparing a plan for him to adjust as soon as possible.
"We know we've another three FIFA breaks from now and before Christmas, so we thought it was better for him - and the team - to prepare himself and focus on the next game against Norwich."
It was the second time in five days Barnsley had conceded five goals, much to the bitter disappointment of manager David Flitcroft.
"I'm not too sure if that defeat was cruel," said Flitcroft.
"The scoreline probably was, but we probably got what we deserved. At times we're not defending individually which has a massive collective effect. I'm disappointed with the way we've conceded the goals."
Flitcroft is now looking for a reaction from all of his players in their next outing at home to Huddersfield on Saturday.
"Football's like life - you either sink or swim, you're a victim or you're not," added Flitcroft.
"The ones who want to fight will fight. I'm not interested in seeing victims. But it's also going to be about how I react and what the players see from me."