Perez, an injured midfielder, had to volunteer for a place between the posts against Independiente Santa Fe, with River heading into their Copa Libertadores group-stage match with just 11 players available.
The team have been hit by a COVID-19 outbreak, which on top of a spate of injuries left River heavily depleted, with the four goalkeepers included in their Libertadores squad testing positive for the virus.
They were not allowed to call up a keeper from their academy, instead relying on 35-year-old Perez in goal while having no substitutes to call on.
Regardless, River took an early two-goal lead – Federico Angileri and Julian Alvarez scoring within the first five minutes – and conceded only once to Kelvin Osorio with 17 minutes remaining.
Perez had only four saves to make in the 2-1 win, despite Colombian side Santa Fe attempting 22 shots and bossing 69 per cent of the possession.
Jersson Gonzalez, Daniel Giraldo, Jorge Ramos and Osorio were all denied by Perez, although their 22 efforts were worth a measly 1.1 expected goals (xG) combined. River scored twice from nine chances with an xG value of 1.6.
"Unfortunately, we had to suffer the contagion issue, but we rescued everyone's heart, manhood and personality," Perez told ESPN.
"We once again demonstrated the group and the kind of people we are. I don't fall too much into everything we've been through, I tried to focus and help the team. There is going to be a lot of talk about this game.
"I was asking questions, practicing with the goalkeeping coach, listening to advice. I lost a bit, but always took the penalty spot as a reference."
River's win sees them top Group D heading into their final game, against Fluminense on Tuesday. The embattled side could still be knocked out, should they lose and third-placed Junior win, but will hope to by then have a bigger squad – and a goalkeeper – to call upon.