The method of multiple-scale perturbation method is developed in a new way to study the propagation of pulses through an optical fiber described by the perturbed Nonlinear Schrödinger's equation. We show that, by newly introducing a proper definition of the phase of the soliton for the first time, one can obtain the corrections to the pulse where the usual soliton perturbation approach fails. A comparison is made with results obtained by other methods in various other works as well as with numerical simulations.