When wielding a monk weapon, your attacks can be extremely precise.
Benefit: You must declare that you are using this feat before you make your attack roll (thus a failed attack roll ruins the attempt). You must use one of the following weapons to make the attack: kama, nunchaku, quarterstaff, sai, and siangham. You can roll your attack roll twice and take the higher result. If one of these rolls is a critical threat, the other roll is used as your confirmation roll (your choice if they are both critical threats). You may attempt a perfect attack once per day for every four levels you have attained (but see Special), and no more than once per round.
Special: A weapon master monk or zen archer monk receives Perfect Strike as a bonus feat at 1st level, even if he does not meet the prerequisites. A monk may attempt an perfect strike attack a number of times per day equal to his monk level, plus one more time per day for every four levels he has in classes other than monk.
So why does the feat have Improved Unarmed Strike as a prerequisite, if you can’t actually use it with unarmed strikes?
Because the feat is intended to be a cool thing that monks can do, and monks get Improved Unarmed Strike automatically (barring an archetype that replaces that feat), so having Improved Unarmed Strike as a prerequisite means it’s easy for monks to learn Perfect Strike but more difficult for other classes. The prerequisite could have been “monk level X,” but that would mean that only monks could take the feat (prohibiting even other martial arts classes or archetypes). Note that the zen archer archetype allows you to use the feat with a bow, which means there’s a precedent for creating an unarmed-combat archetype that modifies the feat for use with other weapons.