We present a plasmonic bandpass filter and refractive index sensor based on perturbed square cavity resonator with slits, which is fed by orthogonally oriented feeding waveguides. The slits provide size reduction in comparison to conventional square cavities and better coupling to the waveguides, while the perturbation and orthogonal feeding scheme provide a pair of transmission zeros in the response of the structure. In that manner, a bandpass filter with high transmission, narrow bandwidth, and excellent selectivity can be readily designed. Moreover, with a slight modification of the structure, a very sharp Fano-like response can be obtained and employed for refractive index sensing with high sensitivity and figure of merit.