When you apply_dtd(), if some ELEMENT or ATTRIBUTE is not defined in the DTD it will be removed from the tree! So, type is removed since the line ATTLIST is wrong.

This code will work:

use XML::Smart;

my $xml = XML::Smart->new;
$xml->{customer}{phone} = "555-1234";
$xml->{customer}{phone}{type} = "home";

$xml->apply_dtd(<<EOF);
<?xml version="1.0" ?>
<!DOCTYPE customer [
<!ELEMENT customer (phone+)>
<!ELEMENT phone (#PCDATA)>
<!ATTLIST phone type CDATA #REQUIRED>
]>
EOF

print $xml->data;
[download]

<?xml version="1.0" encoding="iso-8859-1" ?>
<?meta name="GENERATOR" content="XML::Smart/1.6.8 Perl/5.006001 [MSWin
+32]" ?>
<!DOCTYPE customer [
<!ELEMENT customer (phone+)>
<!ELEMENT phone (#PCDATA)>
<!ATTLIST phone type CDATA #REQUIRED>
]>
<customer>
  <phone type="home">555-1234</phone>
</customer>
[download]