  Any Dark Energy (DE) or Modified Gravity (MG) model that deviates from a
cosmological constant requires a consistent treatment of its perturbations,
which can be described in terms of an effective entropy perturbation and an
anisotropic stress. We have considered a recently proposed generic
parameterisation of DE/MG perturbations and compared it to data from the Planck
satellite and six galaxy catalogues, including temperature-galaxy (Tg), CMB
lensing-galaxy and galaxy-galaxy (gg) correlations. Combining these observables
of structure formation with tests of the background expansion allows us to
investigate the properties of DE/MG both at the background and the perturbative
level. Our constraints on DE/MG are mostly in agreement with the cosmological
constant paradigm, while we also find that the constraint on the equation of
state w (assumed to be constant) depends on the model assumed for the
perturbation evolution. We obtain $w=-0.92^{+0.20}_{-0.16}$ (95% CL; CMB+gg+Tg)
in the entropy perturbation scenario; in the anisotropic stress case the result
is $w=-0.86^{+0.17}_{-0.16}$. Including the lensing correlations shifts the
results towards higher values of w. If we include a prior on the expansion
history from recent Baryon Acoustic Oscillations (BAO) measurements, we find
that the constraints tighten closely around $w=-1$, making it impossible to
measure any DE/MG perturbation evolution parameters. If, however, upcoming
observations from surveys like DES, Euclid or LSST show indications for a
deviation from a cosmological constant, our formalism will be a useful tool
towards model selection in the dark sector.
