It doesn't specifically address this, but I'd apply the general principle of commenting any obscure usage. Of course, one man's obscure is another man's clear.

I ran into this exact thing in an old program I had to modify and added this comment after figuring out what it meant:

######### FINALLY FIGURED OUT WHAT _ IS!!!
# QUOTE:
#
# If any of the file tests (or either the stat() or lstat()  operators
+) are given
# the special filehandle consisting of a solitary underline, then the 
+stat structure
# of the previous file test (or stat operator) is used, saving a syste
+m call.
#
# Isn't that something?  All I gotta do now figure out what or where i
+s the previous
# test??
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