  We present an observational study of the effect of bars on the gas component
and on the star formation properties of their host galaxies in a statistically
significant sample of resolved objects, the $Herschel$ Reference Sample. The
analysis of optical and far--infrared images allows us to identify a clear
spatial correlation between stellar bars and the cold-gas distribution mapped
by the warm dust emission. We find that the infrared counterparts of optically
identified bars are either bar--like structures or dead central regions in
which star formation is strongly suppressed. Similar morphologies are found in
the distribution of star formation directly traced by H$\alpha$ maps. The sizes
of such optical and infrared structures correlate remarkably well, hinting at a
causal connection. In the light of previous observations and of theoretical
investigations in the literature, we interpret our findings as further evidence
of the scenario in which bars drive strong inflows toward their host nuclei:
young bars are still in the process of perturbing the gas and star formation
clearly delineates the shape of the bars; old bars on the contrary already
removed any gas within their extents, carving a dead region of negligible star
formation.
