http://www.playrugbyleague.com.au/wp-content/uploads/2013/06/International-Laws-of-the-Game.pdf
Scoring - Section 6, page 12. Its there in black and white.
Referee unsighted - The referee should not disallow a try because he was not in a position to see the grounding of the ball
Perenara was not in a position to see the grounding of the ball because there were Souths defenders and Matai's body between him and the ball. His no-try ruling, under the laws of the game available on the NRL website, was 100% incorrect. He should have ruled a try and sent it upstairs.
A referee blunder in giving the opposite ruling to what he should have, cost Manly at least 4, possibly 6 points. It was the turning point in the game and its what people have been saying for years will happen, a team could have its season ruined or almost ruined by a referee getting it completely wrong. This could cost Manly 3rd on the ladder at the end of the minor round, and might have cost them a chance at 2nd. That's the price of a ref stuffing things up.