In this paper, matched-field processing is combined with the boundary integral equation method (BIEM) of scattering theory to study a sound source localization problem in a perturbed shallow ocean. It is assumed that there is a known inclusion embedded in a shallow water waveguide. Continuous waves (cw), produced by a sound source, are scattered by the inclusion and then received by a hydrophone array. Because the symmetry of the waveguide has been destroyed by the existence of the inclusion, a proper procedure is required to avoid the mismatching. A numerical scheme is presented that makes use of the separation of the source and the detection array, and greatly reduces the computation. A numerical simulation using this method is presented.