Madjer gave Portugal the lead seven seconds after the opening kickoff and led his team with three for the game.
El Salvador is making its third appearance in the World Cup, having lost all six previous matches, while Portugal is the 2000 champion that has finished lower than fourth only once in the eight tournaments since.
Madjer recorded all three of his goals in the first five minutes, and aided by goals by Belchior and Coimbra, Portugal led 5-0 before Agustin Ruiz converted a penalty for El Salvador 4:03 into the second period.
Portugal increased its advantage to 10-1 before Frank Velasquez scored El Salvador's other goal early in the third.
El Salvador will try again for its first World Cup victory on Saturday when it face tournament debutante Oman.