Two of the most common ways to concentrate and hold post fry perch are 1. herding and holding them into a corner of the pond with a net. Feed training occurs behind the net. 2. Capturing the fry (fingerlings), bring them inside and holding them in a tank that usually has some sort of flow through system bringing in fresh water and flushing wastes & solids. Note in the picture in the link below the fish being feed trained in the tank with a belt feeder. Both methods are not easy and are usually only done on a fairly large scale by hatcheries. Considering the difficulty, complexity, labor involved, and success rate, it is probably cheaper to buy the pellet trained fingerlings. Then raise those by one of various methods to a larger size for release into the pond that has larger predators. For my YP, I like to continue feeding pellets to purchased pellet raised YP to further habuate them to pellets, get them larger and accustomed to my pond conditions before release. This way basically all of them stay feeding almost exclusively on pellets - i.e. lazy fish.
Catching perch as post fry larvae is usually done either with a seine or a light trap depending on the size of the larval fish. Light traps can be simple or complex. Most places do not pellet train until the larvae are about 1"-1.25" long because those are the first size easiest to seine and they are at optimum size or stage for food training.
This link is a common style of belt feeder. The belt is spring loaded & 'cocked' and as the spring unwinds or pulls back it drops feed from the belt into the tank or pond.
http://www.aquaticeco.com/subcategories/2253/Belt-Feeders?green=099B439E-46D3-5A4B-A21B-B22666299806
