I sometimes see or hear these nouns used interchangeably.

Problem:
The nouns “perquisite” and “prerequisite” are not synonyms.

Explanation:
The noun “perquisite” — spelled P-E-R-Q-U-I-S-I-T-E — dates back to the early 1400s and refers to an incidental privilege or payment beyond regular salary or wages.

Many readers may know the informal noun “perk” — spelled P-E-R-K — which was coined in the 1800s as a shortened form of “perquisite”.

The origin of the noun “perquisite” is the Latin word “perquisitus”, which is the participle of the Latin verb “perquirere”, which means to inquire diligently.

This origin makes sense when one knows that the tertiary meaning of “perquisite” is something demanded as a particular privilege, as in “It was a perquisite of royalty.”

In other words, the noun “perquisite” began as a reference to something demanded by royalty, and it evolved to mean something extra given to employees.

The noun “prerequisite” — spelled P-R-E-Q-U-I-S-I-T-E — dates back to the early 1600s and means something prerequisite.

So one then must turn to the meaning of the adjective “prerequisite”, which is required beforehand, with “required” coming from the “requisite” part of “prerequisite” and with “beforehand” coming from the “pre” part of “prerequisite”.

Solution:
Think of the informal noun “perk” to remember the meaning of the noun “perquisite”; think of the parts of the noun “prerequisite” to remember its meaning.