The process of laser beam homogenization in a gas medium placed in front of a thin metallic foil has been studied. Experiments were performed using the Prague Asterix Laser System iodine laser [ Jungwirth et al., Phys. Plasmas 8, 2495 (2001) ] working at 0.44 μm wavelength and irradiance of about 10^15 W/cm^2. Homogenization was detected both by directly analyzing the transmitted laser beam and by studying the shock breakout on the foil rear side. Results show that the gas ionization by the laser pulse induces a strong refraction and produces an effective smoothing of large-scale intensity nonuniformities.