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sub visit_preorder(&$) {
   my ($visitor, $node) = @_;
...
   &visit_preorder($visitor, $l) if defined $l;
   &visit_preorder($visitor, $r) if defined $r;
}

sub visit_preorder {
   my ($node) = @_;
...
      sub { return visit_preorder($r) if defined $r; return; },
   );
}

sub visit_preorder {
   my ($node) = @_;
...
      defined($r) ? sub { return visit_preorder($r); } : (),
   );
}

sub make_iter {
   my $f = shift @_;
...
      return;
   };
}

{
   #      a
...
   }
   print("\n");
}