Javi Gracia's side controlled the game throughout to pick up the three points against a travel-sick Brighton outfit that, despite a busy transfer window, carried little threat in attack to continue a worrying trend on the road.
Pereyra opened the scoring in the 35th minute with the one moment of genuine quality in a first half where Watford huffed and puffed in open play without enjoying too much success.
However, a clever set-piece saw the hosts deservedly take the lead. Jose Holebas's deep corner picked out his unmarked team-mate on the edge of the penalty area, allowing Pereyra to unleash a volley that goalkeeper Mat Ryan could only palm into the top right corner of his net.
Brighton’s goalkeeper had managed to keep out earlier attempts from strike duo Troy Deeney and Andre Gray, while the visitors barely bothered Ben Foster — making his second debut for Watford over a decade on from a successful two-year loan spell — at the other end before the break.
Holebas's left foot should have led to Watford doubling its lead early in the second half, his sumptuous free-kick from deep carelessly headed over by a diving Christian Kabasele, but they did not have to wait much longer for a second to arrive.
Having helped seize possession high up the field, Pereyra drifted in from the left without ever coming under pressure from a Brighton player, allowing him to curl a right-footed shot beyond Ryan.
Substitute Yves Bissouma fired narrowly wide following his introduction but Chris Hughton's team, which finished 2017/2018 with the worst away record in the Premier League, will need to improve on its travels if it's to maintain its top-flight status for a second successive season.