Data is presented for a complex structural and compositional modulation in the perovksite (NaNd)(MgW)O6. This modulation creates a large 14ap X 14ap X 2ap supercell (ap ≈ 3.9 Å is the lattice parameter of the cubic perovskite aristotype) containing ordered regions with doubled (110) d-spacings in the a-b plane separated by two-dimensional periodic antiphase boundaries and accompanied by a nanocheckerboard pattern. Faint periodic modulations in Z-contrast images suggest an associated periodic variation in composition. The presence of a sodium rich impurity implies the composition of the stable perovskite is nonstoichiometic.