Metal halide perovskites have exhibited excellent properties as absorbers in solar cells, but this may simply be the first of many applications for this intriguing class of materials. Here, we report the nonlinear optical response of triiodide (CH₃NH₃PbI₃) and mixed halide (CH₃NH₃PbI_3–xCl_x) perovskite absorbers. The results show that they have large nonlinear refractive index (NRI), 3 orders of magnitude larger than that of silicon. Particularly, the NRI of CH₃NH₃PbI_3–xCl_x is more than two times larger compared to that of CH₃NH₃PbI₃. Meanwhile, both of them have been proven to possess saturable absorption effects with small nonlinear absorption coefficients which indicate that they can maintain excellent absorption under high-intensity irradiation and are favorable to modulators toward large-energy pulsed laser. Taking into consideration the saturable absorption effect, we demonstrated a pulsed laser with the perovskite as a pulse modulator. These results above indicate the potential for perovskites to be employed in nonlinear optoelectronic devices.