We were taught that the difference between the perfect and semi-perfect lies in that the specific heat constants for perfect gases are constant, while for semi-perfect gases they are functions of temperature and temperature only. However, both perfect and semi-perfect gases are a subset of ideal gases for which the main assumption is that the intermolecular forces are negligible. For that reason changes in potential energy associated with these forces are negligible as well.
If that is the case, what changes in a semi-perfect gas with temperature so that the same heat input does not equal the same temperature raise? After all, from my understanding of temperature it is directly proportional to average kinetic energy of the gas molecules, and hence should vary linearly with heat input.