...
while(mouse_left)
{
   wait(1);
}
//mouse is released here
...

var mouse_activated = 0;
...

if(mouse_left && !mouse_activated){
    mouse_activated = 1;//activate the switch
    //The rest of your code here:
}
if(!mouse_left && mouse_activated){
    //your on release code here
    //mouse_activated = 0;//reset te switch
}

void handle_input()
{
while(1)
{
  ... keyboard input

  if(mouse_left) mouse_left_off = 0;
  else
  {
    if(!mouse_left_off) mouse_left_off_event(); // 
    mouse_left_off = 1;
  }
  wait(1);
}
}