Two-dimensional materials support deeply confined and tunable plasmonic modes, which have great potential for achieving device miniaturization and flexible manipulation. In this paper, we propose a diffraction-unlimited system (period≈ λ/20) composed of borophene layer and perovskite grating to investigate the strong coupling between the borophene guiding plasmon (BGP) and perovskite exciton (PE) modes. The resonant energy of the BGP mode could be electrically tuned to match the energy of the PE mode, and a remarkable Rabi splitting is attained under zero-detuning conditions. The splitting energy could reach 230 meV due to the strong field enhancement provided by the BGP mode. Taking advantage of the proposed electrically tunable hybrid system, not only is the reflective amplitude modulation depth up to 99.9%, but the 1.76π phase range modulation is achieved. Furthermore, by increasing the distance between the borophene layer and perovskite grating, a passive parity-time symmetry breaking could be observed with the vanished energy splitting. Our results deepen our understanding of light–matter interactions at the sub-wavelength scale and provide a guideline for designing active plasmonic devices.