Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. In this study, the presence of chlorine is identified in perovskite films synthesized with Cl-containing precursors by means of nanoprobe X-ray fluorescence (Nano-XRF). Additionally, a spatial variation in the Cl incorporation is observed within a given film, and the standard deviation of Cl: I ratio across the film is large. Using Nano-XRF, the Cl incorporation in methylammonium lead iodide perovskite films can be manipulated by precursor stoichiometry ratio.