XML::Twig empty_tags => 'expand' looks like it does what you want. I shortened your XML and fixed the closing "orders tag":
use warnings;
use strict;
use XML::Twig;
my $xml = <<XML;
<orders>
<order>
<alt_number>2222</alt_number>
<state>processed</state>
<user_id>1234</user_id>
<dd_id>54321</dd_id>
<placed_at/>
<total_price>449.0</total_price>
<total_shipping_costs>90.0</total_shipping_costs>
<discount>0</discount>
<dd_status_code>HD</dd_status_code>
<dd_status_desc>ORDER IS ON HOLD UNTIL 11/01/13</dd_status_desc>
<promo_code/>
</order>
</orders>
XML
my $twig = XML::Twig->new(
pretty_print => 'indented',
empty_tags => 'expand',
);
$twig->parse($xml);
$twig->print;
__END__
Output:
<orders>
<order>
<alt_number>2222</alt_number>
<state>processed</state>
<user_id>1234</user_id>
<dd_id>54321</dd_id>
<placed_at></placed_at>
<total_price>449.0</total_price>
<total_shipping_costs>90.0</total_shipping_costs>
<discount>0</discount>
<dd_status_code>HD</dd_status_code>
<dd_status_desc>ORDER IS ON HOLD UNTIL 11/01/13</dd_status_desc>
<promo_code></promo_code>
</order>
</orders>