Recent reports on temporal photoluminescence (PL) intensity fluctuations (blinking) within localized domains of organo-metal lead halide (hybrid) perovskite microcrystals have invoked considerable interest to understand their origins. Using PL microscopy, we have investigated the effect of atmospheric constituents and photoillumination on spatially extended intensity fluctuations in rnethylammonium lead bromide (MAPbBr(3)) perovskite materials, explicitly for micrometer (ca. 1-2 mu m)-sized crystals. Increase in the relative humidity of the ambience results in progressive reduction in the PL intensity, and beyond a threshold value, individual microcrystalline grains exhibit multistate PL intermittency (flickering), which is characteristically different from quasi two-state blinking observed in nanocrystals. Such flickering disappears upon removal of moisture, accompanied by considerable enhancement of the overall PL efficiency. We hypothesize that initiation of moisture-induced degradation marked by the lowering of PL intensity correlates with the appearance of PL flickering, and such processes further accelerate in the presence of oxygen as opposed to an inert (nitrogen) environment. We find that the intrinsic defects not only increase the threshold level of ambient moisture needed to initiate flickering but also modulate the nature of PL intermittency. Our results therefore establish a strong correlation between initiation of material degradation and PL flickering of hybrid perovskite microcrystals, induced by transient defects formed via interaction with the ambience.