Saints boss Mauricio Pochettino made nine alterations to the starting XI from the weekend Premier League draw with Sunderland, and the recalled Steve Davis (2), Emmanuel Mayuka and Gaston Ramirez all got on the scoresheet.
With £36m of summer investments in the shape of Victor Wanyama, Daniel Osvaldo and Dejan Lovren, and several regulars absent from the squad, Perez felt that the fringe players took their opportunity to impress and stake their claims for more first-team action.
Perez told reporters: "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."