This is a follow up to another more broad question. According to Wikipedia on the possibility of reverter,
There is a possibility of reverter when an estate will return to the grantor if a condition is violated. The possibility of reverter can only follow a fee simple determinable. [...] The vesting of the future interest is determinable at the time of the grant, because reverter is automatic if the condition is broken—a possibility of reverter, therefore, is not subject to the Rule Against Perpetuities.
Could someone do a better job of explaining this nuance, especially as it applies to gifts to the state with a reverter clause? How does the reverter clause not create a lasting perpetual interest in the property, ie., a perpetuity?