See this dedicated discussion and please note the difference between INPUT variables and OUTPUT variables: evidently you need to set the latter ones only (by the way MREG would require an integer, not a character string, but it’s an input variable not to be reset).
wvlnsh.f is not a scoring routine, it serves to generate optical photons with a different wavelength as a formerly traveling optical photon is absorbed. Therefore, the resulting angular distribution on a given region boundary is rather provided by the USRBDX and USRYIELD cards.