The effect on neutrino mixing parameters of complex perturbations on the light Majorana neutrino and the charged lepton mass matrices is studied. Because of the near degeneracy of m_1 and $m_2, large corrections to theta_12 are possible even when the perturbations are small, and therefore the initial unperturbed mixing angle theta_12 need not be close to the experimentally observed value. For complex perturbations on the Majorana mass matrix, the phases of the perturbations and the initial Dirac and Majorana phases strongly impact the leading-order corrections to the PMNS mixing angles and phases. Large corrections are also possible with perturbations on the charged lepton mass matrix. Examples using the formalism are given for mu-tau symmetry and non-standard neutrino interactions.
