As verbs the difference between perpend and prepend

is that perpend is (archaic) to ponder, consider while prepend is (computing|linguistics|transitive) to attach (an expression, phrase, etc) to another, as a prefix or prepend can be (rare|transitive) to premeditate; to weigh up mentally.

As a noun perpend

is a brick or stone that has its longest dimension perpendicular to the face of a wall, especially one that extends through the wall's entire thickness.