  We report an optical fiber experiment in which we study nonlinear stage of
modulational instability of a plane wave in the presence of a localized
perturbation. Using a recirculating fiber loop as experimental platform, we
show that the initial perturbation evolves into expanding nonlinear oscillatory
structure exhibiting some universal characteristics that agree with theoretical
predictions based on integrability properties of the focusing nonlinear
Schr\"odinger equation. Our experimental results demonstrate persistence of the
universal evolution scenario, even in the presence of small dissipation and
noise in an experimental system that is not rigorously of an integrable nature.
