Formation of transverse inhomogeneities in the corona of a solid target irradiated by an inhomogeneous main laser pulse and a uniform background pulse was observed experimentally via side-on shadowgraphy. The experimental results were successfully interpreted using a two-dimensional hydrodynamics code. Our simulations identified the onset of sharp contact boundaries between plasma streams of different expansion velocities. The formation and the decay of the contact boundaries is investigated in detail. When the background pulse is used as a laser prepulse, a layer of coronal plasma is formed that enhances main pulse collisional absorption in underdense plasma and creates conditions for an efficient thermal smoothing of the transverse inhomogeneities.