  It has been shown that a Hamiltonian with an unbroken $\cP\cT$ symmetry also
possesses a hidden symmetry that is represented by the linear operator $\cC$.
This symmetry operator $\cC$ guarantees that the Hamiltonian acts on a Hilbert
space with an inner product that is both positive definite and conserved in
time, thereby ensuring that the Hamiltonian can be used to define a unitary
theory of quantum mechanics. In this paper it is shown how to construct the
operator $\cC$ for the $\cP\cT$-symmetric square well using perturbative
techniques.
