Living in almost the same place as scribble and being a night owl who likes to take walks at 2 am I can confirm that the Carlton/Parkville area is very safe on the whole. Late at night (like after 1 am) it's very quiet, but the streets are well-lit, open and carry sound well, so I wouldn't say there's much danger of being assaulted without notice even if there WAS someone after you

Also, most of the crime in Carlton is petty crime; if you don't go into dark alleyways or linger in Tin Alley I seriously doubt you'll run into trouble. Lygon and Swanston are especially safe due to their light levels and openness, and there are usually a few night-shift workers around (which might help deter people from assaulting you).