  Perovskite oxides ABO$_3$ containing heavy B-site elements are a class of
candidate materials to host topological metals with a large spin-orbit
interaction. In contrast to the band insulator BaSnO$_3$, the semimetal
BaPbO$_3$ is proposed to be a typical example with an inverted band structure,
the conduction band of which is composed of mainly the O-2p orbital. In this
study, we exemplify a band-gap modification by systematic structural, optical,
and transport measurements in BaSn$_{1-x}$Pb$_x$O$_3$ films. A sudden
suppression of the conductivity and an enhancement of the weak antilocalization
effect at $x$ = 0.9 indicate the presence of a singular point in the electronic
structure as a signature of the band inversion. Our findings provide an
intriguing platform for combining topological aspects and electron correlation
in perovskite oxides based on band-gap engineering.
