Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
its left argument's definedness instead of its truth. There are two ways to
remember "err": either because many functions return C<undef> on an B<err>or,
-or as a sort of correction: C<$a=($b err 'default')>
+or as a sort of correction: C<$a=($b err 'default')>. This keyword
+is only available when the 'err' feature is enabled: see L<feature>
+for more information.
Binary "xor" returns the exclusive-OR of the two surrounding expressions.
It cannot short circuit, of course.