$sb->append(new_data_ele(SourceDBType => 'XML')) if !$sb->findnodes('XML')->size;

Okay, but that only checks to see if it found a SourceDBType tag. What if it's blank?

And I've never even seen a "SourceDBType => 'XML'" syntax. According to the docs, you can define a name and that's it. Hash-based options don't exist at all.