Installed the module... It actually produces <element/> for empty elements:

use XML::Smart qw( );
my $xml = XML::Smart->new();
$xml->{foo}{bar}{empty} = {};
print scalar $xml->data();
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<?xml version="1.0" encoding="iso-8859-1" ?>
<?meta name="GENERATOR" content="XML::Smart/1.6.8 Perl/5.008008 [MSWin
+32]" ?>
<foo>
  <bar>
    <empty/>
  </bar>
</foo>
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If you use a DTD, then you need to specify the "EMPTY" keyword or else it'll generate <empty></empty> (which is fine cause it means the same thing).

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();
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thanks for the reply/solution. i was trying '' and undef as source values (from the db) for xml::smart, that didnt work the way i expected perl to work. sure, the empty hash, i should have tried that too. thanks for help!