unary operators, but not regarding parsing and parentheses
(see toke.c). Spotted by Eirik Berg Hanssen.
p4raw-id: //depot/perl@31189
argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
Unless otherwise documented, it returns C<1> for true and C<''> for false, or
the undefined value if the file doesn't exist. Despite the funny
-names, precedence is the same as any other named unary operator, and
-the argument may be parenthesized like any other unary operator. The
+names, precedence is the same as any other named unary operator. The
operator may be any of:
-r File is readable by effective uid/gid.