Text::CSV makes reading and writing CSV files very easy. It has a simple example in the synopsis that will probably do what you need just by copying and changing the filename. That takes care of the first half of your problem.
If you're always using exactly the same input XML file you can probably get away with using a regex (regex tutorial)) to search and replace the elements that you want to change-- simply read in the file as text, make the change, and write it out.
If you start making a habit of modifying XML files, especially if they're variable and may be a little unpredictable, you'll probably want to look into XML::TreeBuilder or XML::LibXML
Try putting something together yourself, and if you run into trouble, post back in this thread and show your code and describe what it's doing wrong