Magneto-optical (MO) properties of perforated heterostructures consisting of a thin metallic film perforated with a periodic array of subwavelength slits and a smooth magnetic dielectric are investigated. Rigorous modeling revealed a magnetization-even MO effect determined by the relative change in the intensity of the transmitted or reflected light when the sample is magnetized in-plane perpendicularly to the slits. The effect takes its maximum value when the p-polarized incident light excites quasi-waveguided eigenmodes in the magnetic layer with the phase velocity close to the one of the TE mode in the nongyrotropic planar waveguide.