We numerically investigate perfect soliton crystal (PSC) generation in a sub-harmonic phase-modulated light pumped microcavity. Parts of the modulated pump frequency components are coupled into the microcavity through the Vernier effect between the modulated pump and cavity modes, which forms a chirped and periodic background wave for soliton formation. With improved pump efficiency, PSCs with 5-20 solitons are demonstrated in a microcavity featuring similar to 10 GHz free spectral range. It is further found that the modulator can be removed to decrease the pump power while maintaining PSC state. Our work enriches the dynamics and applications of cavity solitons.