The former Juventus attacker added to his first-half wonder strike with a well-taken finish in the second to earn the Hornets all three points on the opening day.
Brighton, who were the lowest Premier League point earners on their travels last term, struggled to find a route back into the game and ended the contest without a single shot on target.
Watford slowly started to impose themselves on the game, with star man Pereyra skipping inside and having a shot blocked into the path of Troy Deeney, who could not beat Mathew Ryan in the Brighton goal.
Pereyra was wayward from the rebound but, after Dale Stephens came close with a controlled volley up the other end, the Argentine had a goal to celebrate with a first-time effort into the top corner.
Jose Holebas sent a looping cross to the edge of the box, where Pereyra was waiting to smash the ball home.
Watford defender Christian Kabasele was the next to come close, sending a bullet header over the bar when left unmarked, but the hosts did manage to double their lead nine minutes into the second period.
Holebas claimed another assist by playing the ball into Pereyra's path, and the winger bent it out of Ryan's reach after cutting onto his favoured right boot.
There was very little action in the final quarter of the match, however, with Albion offering very little in the final third, meaning a straightforward start to the season for Javi Gracia in the home dugout.
WATFORD (4-4-2): Foster; Holebas, Kabasele, Cathcart, Janmaat; Pereyra (Femenia 87'), Doucoure, Capoue, Hughes (Sema 81'); Deeney, Gray (Success 74')
BRIGHTON & HOVE ALBION (4-5-1): Ryan; Bruno (Bong 26'), Dunk, Duffy, Bernardo; Knockaert, Stephens, Propper, March (Jahanbakhsh 71'), Gross (Bissouma 60'), Murray