We study the dependence on the charm quark mass of the leading-order low-energy constants of the Delta S = 1 effective Hamiltonian, with the aim of elucidating the role of the charm mass scale in the Delta I = 1/2 rule for K arrow right pipi decay. To that purpose, finite-volume chiral perturbation theory predictions are matched to QCD simulations, performed in the quenched approximation with overlap fermions and mu = md = ms. Light quark masses range between a few MeV up to around one third of the physical strange mass, while charm masses range between mu and a few hundred MeV. Novel variance reduction techniques are used to obtain a signal for penguin contractions in correlation functions involving four-fermion operators. The important role played by the subtractions required to construct renormalized amplitudes for mc not = mu is discussed in detail. We find evidence that the moderate enhancement of the Delta I = 1/2 amplitude previously found in the GIM limit mc = mu increases only slightly as mc abandons the light quark regime. Hints of a stronger enhancement for even higher values of mc are also found, but their confirmation requires a better understanding of the subtraction terms.