Perovskite-based flexible nanocomposites were realized by dispersing BaTiO$_3$ and modified monodisperse BaTiO$_3$ in PVDF–HFP matrix. BaTiO$_3$ was modified in situ by the addition of carbon solution, which was prepared electrochemically by using graphite rod. Structural characterization revealed that the decrease in tetragonality due to reductionin particle size of modified BaTiO$_3$ than unmodified BaTiO$_3$. The controlled morphology of treated-BaTiO$_3$ nanoparticles was well dispersed in polymer matrix and exhibited effective dielectric constant.High active surface area of modified BaTiO$_3$ suggested strong interfacial polarization, reduced dielectric loss and induced relaxation as compared to PVDF–HFP/BTnanocomposite. The experimental dielectric behaviour was fitted with theoretical Maxwell–Garnet model and composites followed up to 20 wt.% filler. The polarization effect was further proven by electric modulus studies of nanocomposites in broad frequency (0.1 Hz–1 MHz) and temperature ($−$40 to 130$^{\circ}$C). The results suggested that the shift in relaxation peakstowards higher frequencies with increase in filler content in polymer matrix. Further, a flexible-pressure sensing device was fabricated and evaluated for real applications.