  We compute the Casimir energy which arises in a bi-dimensional surface due to
the quantum fluctuations of a scalar field. We assume that the boundaries are
irregular and the field obeys Dirichlet condition. We re-parametrize the
problem to one which has flat boundary conditions and the irregularity is
treated as a perturbation in the Laplace-Beltrami operator which appears.
Later, to compute the Casimir energy, we use zeta function regularization. It
is compared the results coming from perturbation theory with the WKB method.
