  Context. In the past few years, there has been a rise in the detection of
streamers, asymmetric flows of material directed toward the protostellar disk
with material from outside the star's natal core. It is unclear how they affect
the process of mass accretion, in particular beyond the Class 0 phase. Aims. We
investigate the gas kinematics around Per-emb-50, a Class I source in the
crowded star-forming region NGC 1333. Our goal is to study how the mass infall
proceeds from envelope to disk scales in this source. Results. We discover a
streamer delivering material toward Per-emb-50 in H$_2$CO and C$^{18}$O
emission. The streamer's emission can be well described by the analytic
solutions for an infalling parcel of gas along a streamline with conserved
angular momentum, both in the image plane and along the line of sight
velocities. The streamer has a mean infall rate of $1.3 \times 10^{ -6}$ M$_{
\odot}$ yr$^{ -1}$, $5 -10$ times higher than the current accretion rate of the
protostar. SO and SO$_2$ emission reveal asymmetric infall motions in the inner
envelope, additional to the streamer around Per-emb-50. Furthermore, the
presence of SO$_2$ could mark the impact zone of the infalling material.
Conclusions. The streamer delivers sufficient mass to sustain the protostellar
accretion rate and might produce an accretion burst, which would explain the
protostar's high luminosity with respect to other Class I sources. Our results
highlight the importance of late infall for protostellar evolution: streamers
might provide a significant amount of mass for stellar accretion after the
Class 0 phase.
