I think you are tackling this problem the wrong way. You are using an XML tools to work on non-XML data. XML::Twig can help there, but as much as it costs me to admit it, it can't perform miracles ;--(

Where does the data come from? The proper time to add tags to the abc bcd def lines is _before_ you create the XML, or before these lines get into the XML. You don't have necessarily to create the whole row/col structure, but at least tag each cell. This will be trivial in a non-XML context, and this way you won't have to fight with the data.

If this is really the content you have to work with, then just add a first pass, where you add tags to those columns, by just taking the content of your thead element, using regexps to split it, and just print to create the proper XML: you will have to deal with escaping markup characters but that will be simpler than what you are trying to do at the moment.