Answer
$QE \ell$.
Work Step by Step
The electric field applies a force of magnitude QE on each charge. The lever arm for each force is the distance r from the pivot point to each charge, which is $\ell /2$.
Because the field is perpendicular to the rod, the torque caused by each force is $\tau=Fr=QE\ell /2$. Both torques rotate the rod in the same way, so the torques add together.
The net torque is $\tau_{net}=2Fr=2(QE\ell/2) =QE \ell$.