Northbridge is the place to go in Perth to find nightlife. A large number of pubs, clubs and great little asian restaurants are all located within a couple of streets of each other here and it's entirely possible to spend a night wandering from one to the other. Mind you, if you do that you might also spend quite a bit of your night queuing to get into places! Northbridge has also had a bit of a reputation in past years as being somewhere a little unsafe and as a result there is a huge police presence. As with any area where you have large numbers of people drinking there are occasional fights but I have always felt very safe there walking alone at night, although I wouldn't walk off the main streets. Exercise some caution and you'll be fine and the enjoy the nightlife in Perth!