use XML::Smart qw( );
my $xml = XML::Smart->new();
$xml->{foo}{bar}{empty} = {};
print scalar $xml->data();

##</code><code>##

<?xml version="1.0" encoding="iso-8859-1" ?>
<?meta name="GENERATOR" content="XML::Smart/1.6.8 Perl/5.008008 [MSWin32]" ?>
<foo>
  <bar>
    <empty/>
  </bar>
</foo>

##</code><code>##

use XML::Smart qw( );
my $xml = XML::Smart->new();
$xml->{foo}{bar}{empty} = {};
$xml->apply_dtd(<<'__EOI__');
<!DOCTYPE foo [
<!ELEMENT foo (bar*)>
<!ELEMENT bar (empty*)>
<!ELEMENT empty EMPTY>
]>
__EOI__
print scalar $xml->data();