That's nitpickery, *nothing* can output *perfect* square waves. A maximum step from say 0V to 5V at 44kHz is ~0.22V/µs (if I didn't mess that up, I'm still not quite awake ^^) which is definitely doable with a good DAC. The first one I found right now can do 35V/µs for example.
What *I* was...