A procedure has previously been developed for the iterative construction of invariants associated with magnetic field-line Hamiltonians that consist of an axisymmetric zeroth-order term plus a non-axisymmetric perturbation. Approximate field-line invariants obtained with this procedure are used to examine the topological properties of magnetic field lines in a parabolic-current MHD equilibrium that was slightly perturbed from axisymmetry in the limit of a periodic cylindrical configuration. Excellent agreement between Poincaré maps and the level curves of the first-order invariant is found for small perturbations. A means of circumventing the ‘small-divisor problem’ in some cases is identified and implemented with outstanding results. These results indicate that this perturbation method can have valuable consequences for future investigations of magnetic field-line topology.