Tonda Eckert’s side took the lead after 12 minutes as Welington made a dash down the left and lofted a cross into the area where Harwood-Bellis rose highest and headed the ball into the top right-hand corner.
Fellows came close to a second just minutes later, but his curled effort from the edge of the box flew narrowly wide of the far post.
The hosts then had a decent chance of their own as shots from Alfie Devine and Ben Whiteman were blocked at close range before Saints scrambled clear.
Paul Heckingbottom’s side came close to an equaliser after 30 minutes when Liam Lindsay headed just wide of the target.
But Southampton doubled their lead early in the second half when Cameron Archer fizzed a pass into Stewart who swivelled on the edge of the box before slotting the ball into the bottom corner with his left foot.
The Lilywhites managed to get a goal back when goalkeeper David Cornell made a sharp stop at his near post before sending a ball over the top for Dobbin to chase.
Peretz came out of his box in an attempt to clear the danger, but misjudged the flight of the ball and slipped as he attempted to correct himself, allowing Dobbin to collect the ball and roll the ball into an empty net.
But Larin came off the bench to score in injury time and secure the three points for the Saints, tapping Edozie’s cross into the net from close range.
Ryan Manning struck the post with the final kick of the game with a free-kick from just outside the area.
