I think you want the SupressEmpty option. Copy/pasted from the XML::Simple CPAN page:

SuppressEmpty => 1 | '' | undef # in+out - handy

This option controls what XMLin() should do with empty elements (no attributes and no content). The default behaviour is to represent them as empty hashes. Setting this option to a true value (eg: 1) will cause empty elements to be skipped altogether. Setting the option to 'undef' or the empty string will cause empty elements to be represented as the undefined value or the empty string respectively. The latter two alternatives are a little easier to test for in your code than a hash with no keys.