A "perfect block" occurs when three opponents' available squares are occupied in one move.What this means is blocking all 3 opponent's options by placing a single tile. As such, you can only get this in a Classic 4 game in Tournament or Online mode, local Quick Play mode will not work.
In a Classic 4 game, head for the center and evaluate the board carefully. With some luck, you will be able to spot a chance and place a tile in the center that blocks all your AI opponents. You will likely get this naturally while going through Tournament Mode.
Tip by FMR
Here's a tip to get it. When you play against 3 computers, choose the 3rd position, the red one. Then on the 3rd or 4th round, you'll have the best chance to make a perfect block has most of the time, the 3 or 4 first moves target the center. Choose the right piece to block the 3 computers. Quite easy to get it.