  A large data set of 78 solar proton events observed near the Earth's orbit
during 1996-2011 is investigated. An East-West longitudinal (azimuthal)
asymmetry is found to exist in the distribution of flare sources of solar
proton events. With the same longitudinal separation between the flare sources
and the magnetic field line footpoint of observer, the number of the solar
proton events originating from solar sources located on the eastern side of the
nominal magnetic footpoint of observer is larger than the number of the solar
proton events from solar sources located on the western side. We emphasize the
importance of this statistical investigation in two aspects. On the one hand,
this statistical finding confirms our previous simulation results obtained by
numerically solving five-dimensional Fokker-Planck equation of solar energetic
particle (SEP) transport. On the other hand, the East-West longitudinally
(azimuthally) asymmetric distribution of solar proton events accumulated over a
long time period provides an observational evidence for the effects of
perpendicular diffusion on the SEP propagation in the heliosphere. We further
point out that, in the sense of perpendicular diffusion, our numerical
simulations and statistical results of SEP events confirm each other. We
discuss in detail the important effects of perpendicular diffusion on the
formation of the East-West azimuthal (longitudinal) asymmetry of SEP
distribution in two physical scenarios, i.e., 'multiple SEP events with one
spacecraft' and 'one SEP event with multiple spacecraft'. A functional relation
I_{max}(r)=kr^{-1.7} quantifying the radial dependence of SEP peak intensities
is obtained and utilized in the analysis of physical mechanism. The
relationship between our results and those of Dresing et al. is also discussed.
