All the goals came inside the first quarter of an hour, with Etienne Capoue volleying the hosts ahead after just 32 seconds.
Pereyra curled past a helpless Ron-Robert Zieler on 12 minutes before Riyad Mahrez replied quickly from the penalty spot.
Leicester couldn’t draw level as they continued to struggle this season, while the Hornets keep up their brilliant start to the Premier League season.
Walter Mazzarri’s side came into the game off the back of an embarrassing 6-1 defeat to Liverpool last weekend, but they didn’t show any signs of a hangover against the Foxes.
The hosts went and took the lead inside the first minute when Pereyra broke down the left and delivered a cross when he seemed to be in a tight predicament.
Hornets race into two-goal lead
Troy Deeney flicked the delivery down to Capoue, who volleyed into the net for his fifth goal of the season.
Leicester seemed shell-shocked and Watford took full advantage of that as they went and doubled their advantage on 12 minutes.
It was a moment of real beauty from Pereyra, who turned sharply past Danny Drinkwater before curling brilliantly into the far corner past a helpless Zieler.
This was the strike that provoked a reaction from the sleeping giant, Leicester, and the Foxes had a chance to get back into the game just three minutes later.
Foxes reply from the penalty spot
Jamie Vardy was brought down in the penalty area by Miguel Britos and Mahrez stepped up and slotted the spot kick straight down the middle.
Chances were few and far between all the way until the hour mark thereafter, neither side could create an opening during a scrappy period of the game.
Just after the hour mark, Watford had a chance to restore their two-goal lead when Pereyra came close to netting his second of the game.
Daryl Janmaat nodded down Nordin Amrabat’s cross down to the playmaker, who forced Zieler into making a terrific save.
Leicester condemned to another away defeat
Claudio Ranieri introduced Ahmed Musa late on and the forward was denied before Robert Huth’s effort was collected by Heurelho Gomes.
Leicester huffed and puffed, but couldn’t blow the Watford defence down, and they were condemned to their fifth away defeat of the season.
The Foxes now sit just outside the relegation area by two points as they are set to return to Champions League action against Copenhagen through the week.