Perotti collected the ball on the left side edge of the penalty area but found a Plzen defender blocking his way to goal.
He initially looked as if he would try to get to the byline but then wrapped his right foot around the back of his standing left leg and chipped the ball into the far corner.
The Argentine forward's celebrations were muted, immediately suggesting that the goal was a fluke.
"The truth is that I tried to cross it," Perotti admitted after the 4-1 win which took Roma to the knockout stages.
"But it's good that it went in – for the team, because it killed off the game, and for me because until then I had only scored from penalties."
Perotti had previously scored five goals in all competitions this season, all from the penalty spot.
(Writing by Brian Homewood in Berne; editing by Sudipto Ganguly)
