  An interaction between dark matter and dark energy, proportional to the
product of their energy densities, results in a scaling behavior of the ratio
of these densities with respect to the scale factor of the Robertson-Walker
metric. This gives rise to a class of cosmological models which deviate from
the standard model in an analytically tractable way. In particular, it becomes
possible to quantify the role of potential dark-energy perturbations. We
investigate the impact of this interaction on the structure formation process.
Using the (modified) CAMB code we obtain the CMB spectrum as well as the linear
matter power spectrum. It is shown that the strong degeneracy in the parameter
space present in the background analysis is considerably reduced by considering
\textit{Planck} data. Our analysis is compatible with the $\Lambda$CDM model at
the $2\sigma$ confidence level with a slightly preferred direction of the
energy flow from dark matter to dark energy.
