This could work, if you have the older CNC shield. On the new V3.51 you need to connect IN to Spindle Enable (pwm) instead of Z+.
But I would not recommend to directly connect L, P and G. This is not save!!
It would fire the laser when there is any voltage higher than 0V on the IN pin. When the Arduino is off or rebooting, this pin can float and therefore fire the laser.
It’s saver to use the P pin for “Laser Enable”, door locks and water flow sensor (in series), as it is on the original wiring.