  Electron-phonon coupling was believed to govern the carrier transport in
halide perovskites and related phases. Here we demonstrate that
electron-electron interaction plays a direct and prominent role in the
low-temperature electrical transport of compressed CsPbI3 and renders Fermi
liquid (FL)-like behavior. By compressing {\delta}-CsPbI3 to 80 GPa, an
insulator-to-metal transition occurs, concomitant with the completion of a
sluggish structural transition from the one-dimensional (1D) Pnma ({\delta})
phase to a 3D Pmn21 ({\epsilon}) phase. Deviation from FL behavior is observed
in CsPbI3 upon entering the metallic {\epsilon} phase, which progressively
evolves into a FL-like state at 186 GPa. First-principles density functional
theory calculations reveal that the enhanced electron-electron coupling is
related to the Cs-involved electron transfer and sudden increase of the 5d
state occupation of the high-pressure {\epsilon} phase. Our study presents a
promising strategy for tuning the electronic interaction in halide perovskites
for realizing intriguing electronic states.
