A mechanism which relates the upturn of the perpendicular upper critical magnetic field H_c2^{perpendicular} (T) in layered superconductors and thin films with the structural inhomogeneity in the bulk of the sample, provided that the local critical temperature T_c* inside the inhomogeneity is higher than in the rest of the sample (T_c) is proposed. Within the Ginzburg-Landau approach an equation which describes two types of experimentally observed nonlinearities in H_c2^{perpendicular} (T) near T_c for ISN (insulator-superconductor-normal metal) and NSN layer configurations, is found. In the NSN case a crossover from the linear branch H_c2^{perpendicular}(T) propto (T_c - T), for fields H <= H_m, to the nonlinear branch with the upturn, if H > H_m, takes place. The crossover field H_m is inversely proportional to the local enhancement of the critical temperature (T_c* - T_c) and the distance R to the surface (the nearest surface, in case of a thin film). In the ISN case the upturn holds for H < H_m, whereas for higher fields H_c2^{perpendicular} (T) crosses over to the linear branch. In the ISI case the H_c2^{perpendicular} (T) is a linear function.

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