If you're using XML::RPC, this should work:

sub as_string { 
   my $val = shift;
   return sub {   { string => $val };   };
}
...

$xmlrpc->call( 'gimmeallyourmoney', { cardnumber => as_string( 12345 )
+ } );
[download]

Which generates:

<?xml version="1.0" encoding="UTF-8" ?>
<methodCall>
<methodName>gimmeallyourmoney</methodName>
<params>
<param>
<value>
<struct>
<member>
<name>cardnumber</name>
<value>
<string>12345</string>
</value>
</member>
</struct>
</value>
</param>
</params>
</methodCall>
[download]

Update: Added patch to XML::RPC-wishlist.