A voltage/current regulator is a pretty steep low-pass filter. It's not something you can use as a "driver" for a varying signal. You should drive that current regulator directly from the power rail.
What you want is to "and" each row strobe signal with the PWM. You can do this using 8 discrete AND gates -- something like 2x 4081 chips (if you can still get discrete gates -- haven't checked in many years!) and tie the output of each AND gate to the gate input of your N-channel switch.
You can also do this by adding a second N-channel MOSFET in series with the first one for each row, and driving the gate of all those transistors with the PWM output. The problem with THAT is that you only get < 40 mA out of the Arduino, so driving 8 MOSFET input gates may not be fast enough (we're talking microseconds here -- it may also be fast enough.) If that's the case, use another buffer device, such as complementary pair of P-channel and N-channel MOSFET devices (BS170, BS250P) or a dedicated output buffer like the 4050.