function main()
{
   ...
   on_space=player_jump;  //call this just once
   ...
}

function player_jump()
{
   player_entity.z += 10;  //ME is not valid here
}

action your_action()
{
   var space_pressed = 0;   //before WHILE loop starts
   ...
   if(key_space==0)    
   {
      space_pressed = 0;
   }
   else    
   {
      space_pressed = 1;
      if(space_pressed==0)   my.z += 10;
   }
   ...
}