Chasing a fifth successive clean sheet and win, the hosts were taken to the cleaners in the first half, with Pompey netting three times in an eight-minute period in the first quarter following an abject, damning and rudderless opening.
Creditably, Barnsley - with Callum Styles, Sam Cosgrove and Owen Dodgson brought on at the interval and Corey O'Keefe also making an impression - improved markedly on the restart.
Barry Cotter pulled a goal back early in the second period and a 77th-minute header from Styles set up a grandstand finish.
But Pompey held out for three points and moved top of the table on goal difference following a 3-2 verdict.
On his triple change at the break, Collins, who brought off John McAtee, Jon Russell and Nicky Cadden, said: "It was easier to make that decision based on who we knew we could bring on. I didn't want to make change for change's sake, but changes I felt we needed.
"We needed someone of Sam's stature and Styles's energy and the other players brought the things we expected them to do."
Commenting on his side's display, he continued: "It is important we don't take anything out of proportion and I say that with both halves of football.
"I think the one minute, between the first and second goal, really affected us and within one minute, you are 2-0 down and the game has not had chance to take shape.
"The second goal was schoolboy stuff and not necessarily tactical, but decision-making and we need to look at making sure that we don't make that decision (again) in basic moments of the game.
"Similarly, in the second half, there were so many positives. But equally, at 3-0 down, there's only one thing to do and that's to play with purpose.
"I learnt a lot and there's a couple of things we need to really look at as we can't start a game like that in terms of giving away a couple of basic goals.
"But there were positives about the team and we went right to the very end and some of the situations we got into, we could have easily been sat here with a point."
Collins received a booking in the first half following his protests after referee Lewis Smith did not point to the penalty spot after Cotter went down in the box.
The Lancashire official incurred the wrath of the Oakwell crowd for most of the night.
Collins said: "With those two decisions, I just thought, based on the decision given against us as a penalty, that it looked like a penalty.
"But I have been reminded that Barnsley don't get penalties. We're due maybe one a season."