A quadrilateral with congruent and perpendicular diagonals is not necessarily a square. A square is a special type of quadrilateral where all sides are equal in length and all angles are right angles. While a square does have congruent and perpendicular diagonals, there are other quadrilaterals that also have congruent and perpendicular diagonals, such as a rectangle or a rhombus. Therefore, the statement is false.