A quite unusual diffuse scattering phenomenology was observed in the single-crystal x-ray diffraction pattern of cubic perovskite BMT (BaMg_1/3Ta_2/3O₃).The intensity of the scattering is parametrized as a set of cubelike objects located at the centers of reciprocal space unit cells, resembling very broad and cubic-shaped (1/2,1/2,1/2) satellites. BMT belongs to perovskites of formula A B′_1/3B′′_2/3O₃ (A = Mg, B′ = Ta, B′′ = Mg). The cubes of the intensity can be attributed to the partial correlations of the occupancies of the B site. The pair correlation function is the Fourier transform of the diffuse scattering intensity and the latter's idealized form yields the unusual property of a power-law correlation decay with distance. Up to now this has been observed only in a few exotic instances of magnetic order or nematic crystals. Therefore it cannot be classified as a short-range order phenomenon, as in most situations originating diffuse scattering. A Monte Carlo search in configuration space yielded solutions that reproduce faithfully the observed diffuse scattering. Analysis of the results in terms of the electrostatic energy and the entropy points to this phase of BMT as a metastable state, kinetically locked, which could be the equilibrium state just below the melting point.