Among reliability studies on perovskite photovoltaics (PV) cells and modules, partial shading degradation is a crucial and under-investigated topic. In the present work, we use a combination of mapping electroluminescence (EL), photoluminescence (PL), and illuminated lock-in thermography (ILIT) to gain insight into the reverse bias degradation mechanisms induced by partial shading on a monolithically interconnected module. Spatial inhomogeneities across the cell length are shown to play an important role in the degradation. A perovskite module was subjected to partial shading, causing, in the lower region of the shaded cells, a PL signal intensity increase and EL decrease. We suggest the formation of a barrier at one of the perovskite/transport layer interfaces, preventing both carrier extraction in PL and carrier injection in EL. A simple model for the current flow in the presence of the barrier can satisfactorily explain the EL, PL, and ILIT behavior and point to some possible propagation mechanisms. In summary, we show that studying partial shading degradation at module level draws a more complex and realistic picture of the interplay between material and electrical parameters than cell-level studies. We also demonstrate that luminescent and thermal imaging techniques can be combined to draw meaningful conclusions on the degradation mechanisms, their formation, and propagation.