  We demonstrate a novel technique for calibrating the energy scale of the XMM
EPIC-pn detector, which allows us to measure bulk flows in the intracluster
medium (ICM) of the Perseus and Coma clusters. The procedure uses the
instrumental lines present in all observations, in particular, Cu-Ka. By
studying their spatial and temporal variations, in addition to incorporating
calibration observations, we refined the absolute energy scale to better than
150 km/s at the Fe-K line, a large improvement over the nominal accuracy of 550
km/s. We then mapped the bulk motions over much of the central 1200 and 800 kpc
of Perseus and Coma, respectively, in spatial regions down to 65 and 140 kpc
size. We cross-checked our procedure by comparing our measurements with those
found in Perseus by Hitomi for an overlapping region, finding consistent
results. For Perseus, there is a LoS velocity increase of 480+-210 km/s
(1sigma) 250 kpc east of the nucleus. This region is associated with a cold
front, providing direct evidence of the ICM sloshing in the potential well.
Assuming the intrinsic distribution of bulk motions is Gaussian, its width is
214+-85 km/s, excluding systematics. Removing the sloshing region, this is
reduced to 20-150 km/s, which is similar in magnitude to the Hitomi line width
measurements in undisturbed regions. In Coma, the line-of-sight velocity of the
ICM varies between the velocities of the two central galaxies. Maps of the gas
velocity and metallicity provide clues about the merger history of the Coma,
with material to the north and east of the cluster core having a velocity
similar to NGC 4874, while that to the south and west has velocities close to
NGC 4889. Our results highlight the difference between a merging system, such
as Coma, where we observe a ~1000 km/s range in velocity, and a relatively
relaxed system, such as Perseus, with much weaker bulk motions. [abridged]
