We study a modified backwards-facing step flow, with the addition of two different splitter plates; one is a baseline, impermeable plate and the second a perforated one. An experimental investigation is carried out for a turbulent reattaching shear layer downstream of the two plates. The proposed setup is a model configuration to study how the plate characteristics affect the separated shear layer, and also how turbulent kinetic energies and large-scale coherent structures are modified. Measurements show that the perforated plate changes the mean flow field, mostly by reducing the intensity of backflow close to the bottom wall. Disturbance amplitudes are significantly reduced up to 5 step-heights downstream of the trailing edge of the plate, more specifically in the recirculation region. A loudspeaker is then used to introduce phase-locked, low-amplitude perturbations upstream of the splitter plates, and phase-averaged measurements allow a quantitative study of large-scale structures in the reattaching shear-layer. The evolution of such coherent structures is evaluated using linear stability theory, comparing the eigenfunction of the Kelvin-Helmholtz mode to the experimental results. We observe a close match of linearstability eigenfunctions with phase-averaged amplitudes for the two tested Strouhal numbers. The perforated plate is found to reduce the amplitude of the Kelvin-Helmholtz coherent structures in comparison to the baseline, impermeable plate, a behavior consistent with the predicted amplification trends from linear stability. Such perforated plates can be seen as a possible technological solution for passive flow control, reducing the intensity of turbulence and recirculation in the flow field.