I'm afraid there may be a bug in the AnyData convert(). Try to just do it with SQL:

my $dbh = DBI->connect('dbi:AnyData(RaiseError=>1):');
$dbh->ad_catalog('myXml','XML','old.xml');
$dbh->ad_catalog('myCsv','CSV','new.csv');
$dbh->do("CREATE TABLE myCsv AS SELECT * FROM myXml");
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Trizor is correct that you need to select a portion of the XML that fits into a table-like structure. AnyData uses XML::Twig under the hood so it does allow you to pick and choose (to a certain extent) what part of the XML file to convert to CSV. You may have to experiment with the col_names and other attributes, see AnyData::Format::XML.