For perturbations of the Callias and Anghel type the L2-index of the perturbed Dirac operator on a Spin c -manifold is realized as the result of pairing an element in K -homology with an element of compactly supported K -cohomology. This is achieved by putting the problem of calculating the Fredholm index of the perturbed Dirac operator in the framework of KK-theory and using the identification of K-groups with KK-groups. The formula for the Fredholm index is given in terms of topological data of the Spin c-manifold and the structure of the perturbation.