Inorganic metal-halide perovskite materials pave the way for many applications ranging from optoelectronics to quantum information due to their low cost, high photoluminescence and energy conversion efficiencies. However, light-induced bandgap instability due to ion migration in mixed-halide perovskites remains a significant challenge to the efficiency of optoelectronic devices. Thus, we combined hyperspectral fluorescence microspectroscopy and computational methods to understand the underlying transition mechanism between phase reconstruction and segregation in CsPbBr