The ability to initialize and control long-lived spin populations in lead halide perovskites is essential to enable their use in spintronics technologies. Here, we show that the interlayer exciton observed in the photoluminescence spectrum of a (BA)₂PbI₄/WSe₂ monolayer heterostructure below the intralayer exciton transitions exhibits circular dichroism, whose sign can be modulated by tuning the excitation energy. Equilibrium and transient absorption measurements reveal an additional absorptive feature between the A and B excitons of WSe₂ with hybrid character. This resonance arises from hybridized valence band states of the two materials and plays a key role in controlling the helicity of the interlayer exciton emission. The tunable spin polarization demonstrated here, with the WSe₂ monolayer effectively acting as a tunable spin filter, represents an important step toward the use of 2D perovskites in opto-spintronics applications.