What is _ (single underscore) in Perl without $ or @?

I have some code like this:

if (!-d _ || !-o _ || ($vncUserDirUnderTmp && ($mode & 0777) != 0700)) {
    raise Exception("$prog: Wrong type or access mode of $vncUserDir.\n")
}

I could find information about @_ and $_ in Perl but not about _ alone. What does it mean in -d _ and -o _ here?