Yes, but no.
PWM is not a form of current control, though it can be thought of as sort of an "averaging" current control.
PWM simply adjusts how long the laser is on, but in very short timeframes (A couple of Khz), so that the "perceived" or "average" power is lower. But, your laser tube still takes the full power for that extremely short time, which can put stress on the tube.
However, you may be able to get away with using a ballast resistor to dump off some of the extra power, but you need to be careful with how much you're dissipating through it.
You can run larger tubes on smaller PSU's, but generally a bad idea the other way around.