  Aim of the paper is to provide a method to analyze the behavior of
$T$-periodic solutions $x_\eps, \eps>0$, of a perturbed planar Hamiltonian
system near a cycle $x_0$, of smallest period $T$, of the unperturbed system.
The perturbation is represented by a $T$-periodic multivalued map which
vanishes as $\eps\to0$. In several problems from nonsmooth mechanical systems
this multivalued perturbation comes from the Filippov regularization of a
nonlinear discontinuous $T$-periodic term. \noindent Through the paper,
assuming the existence of a $T$-periodic solution $x_\eps$ for $\eps>0$ small,
under the condition that $x_0$ is a nondegenerate cycle of the linearized
unperturbed Hamiltonian system we provide a formula for the distance between
any point $x_0(t)$ and the trajectories $x_\eps([0,T])$ along a transversal
direction to $x_0(t).$
