Ultrafast terahertz (THz) modulation characteristics of organolead halide perovskite films (CH3NH3PbI3 and CH3NH3PbI3-xClx) were investigated on picosecond time scales using time-resolved THz spectroscopy. Upon photo-excitation, a transient decrease in THz transmission was observed. Compared with CH3NH3PbI3, CH3NH3PbI3-xClx showed a better modulation depth (10%) within the range of the photo-excitation powers used. The mechanism underpinning this photoconductive ultrafast response was determined by measuring the transmission properties and calculating the carrier density. The larger crystalline bulk of the CH3NH3PbI3-xClx film produced higher carrier densities than the CH3NH3PbI3 film. These results demonstrate that CH3NH3PbI3-xClx films are promising materials for developing high-performance THz modulators and ultrafast switchable THz photoelectric devices.