This paper is focused on a perturbation method that allows designing dual-band impedance matching networks for applications where the frequency bands are very close. Such networks are based on resonant type metamaterial transmission lines, namely, artificial lines consisting on a host propagating medium loaded with electrically small resonators. These resonators can be tailored in order to obtain the design requirements of the network, which are not achievable through conventional lines. The main aim of this work is to demonstrate that a perturbation method can be successfully used when the operating frequencies are very close.