Code:

---

var phaser_decreasing = 0;

function  init_player_startup()
{
    while (player == null) {wait (1);}
    player.phaser = 74;
    while (1)
    {
       if(phaser_decreasing == off)
       {
              player.phaser += 0.4 * time_step; // increase power slowly
              player.phaser = min(74, player.phaser); // limit phaser power 74
       }
       wait (1);
    }
}

function beam_weapons
{
	while ((mouse_left == 1))
	{
		phaser_decreasing = on;
		player.phaser -= 3*time_step;
		snd_play (beam_wav, 50, 0);
		wait(1);
	}
	phaser_decreasing = off;
}

on_mouse_left = beam_weapons;

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By the way, don't use the TIME variable anymore. It's kinda not stylish these days. use time_step instead. Use it in the exact same way, just a different name. I replaced your TIME variables in the functions above with time_step already.