  Compositional engineering of the optical properties of hybrid
organic-inorganic lead halide perovskites is one of the cornerstones for the
realization of efficient solar cells and tailored light-emitting devices. We
study the effect of compositional disorder on coherent exciton dynamics in a
mixed FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ perovskite crystal using photon
echo spectroscopy. We reveal that the homogeneous linewidth of excitons can be
as narrow as 16$\mu$eV at a temperature of 1.5K. The corresponding exciton
coherence time of $T_2=83$ps is exceptionally long being attributed to the
localization of excitons due to variation of composition at the scale of ten to
hundreds of nanometers. From spectral and temperature dependences of the two-
and three-pulse photon echo decay we conclude that for low-energy excitons,
pure decoherence associated with elastic scattering on phonons is comparable
with the exciton lifetime, while for excitons with higher energies, inelastic
scattering to lower energy states via phonon emission dominates.
