Here's why:

* Perchlorate is the polyatomic ion ClO₄⁻.

* Chlorine (Cl) has a typical oxidation state of +7 in this ion.

* Oxygen (O) has a typical oxidation state of -2.

* The overall charge needs to balance. Since there are four oxygen atoms with a -2 charge each, the total negative charge from oxygen is -8. To balance this, the chlorine atom needs to have a +7 charge.

* Combining these charges (+7 - 8 = -1), the overall charge on the perchlorate anion is -1.