Link to undef for consistency

pod/perlfunc.pod

@@ -3370,10 +3370,10 @@ X<glob> X<wildcard> X<filename, expansion> X<expand>
 =for Pod::Functions expand filenames using wildcards
 
 In list context, returns a (possibly empty) list of filename expansions on
-the value of EXPR such as the standard Unix shell F</bin/csh> would do.  In
+the value of EXPR such as the standard Unix shell C</bin/csh> would do. In
 scalar context, glob iterates through such filename expansions, returning
-undef when the list is exhausted. If EXPR is omitted, L<C<$_>|perlvar/$_>
-is used.
+L<C<undef>|/undef EXPR> when the list is exhausted. If EXPR is omitted,
+L<C<$_>|perlvar/$_> is used.
 
     # List context
     my @txt_files  = glob("*.txt");