My idea was to consider $f \colon X \to Y$ such that $a_Y^\ast \mathbf Q \cong IC_Y$ but $a_X^\ast \mathbf Q \not\cong IC_X$, where $a$ denotes the map to a point and all functors are derived. Then $a_X^\ast \mathbf Q = f^\ast IC_Y$ can not be perverse, so $f^\ast$ is not t-exact. 

For a concrete example let $X$ be two copies of $\mathbb A^1$ glued at a point, let $f\colon X \to Y \cong \mathbb A^1$ be the quotient by the involution switching the two copies.