We study the positivity of the second shape derivative around an equilibrium for a
functional defined on exterior domains in the plane and which involves the perimeter of the domains
and their Dirichlet energy under volume constraint. We prove that small analytic perturbations
of circles may be stable or not, depending on the positivity of a simple and explicit two-variable
quadratic form. The approach is general and involves a numerical criterion of independent interest
for the positivity of a quadratic form on a given hyperplane.