To turn the XML into useful data structures (that is such DSs that are easy to use, not those that exactly match the layout of the XML giving you a hard to navigate maze of generic objects) you might use XML::Rules. Have a look at Simpler than XML::Simple for a comparison with XML::Simple.

Turning that structure back into XML in the original format is a wee bit harder. Not sure what exactly do you need, but you might use the XML::Rules' filter mode and replace the (few) values you want to change by the filter, but it's not going to be very generic. I started work on some template based way to turn those tweaked data structures to XML in a specific format, but that's far from completion :-(

If instead of a DTD Google provides a XML Schema, you may want to have a look at XML::Compile. That should be a way to do what you are after.