Either of these modules can pretty much just be dropped into the above script by modifying two lines of the script's code

Actually, it shouldn't be necessary to modify the code at all. Your sample code uses XML::SAX::ParserFactory which will use the system default SAX parser (as defined in lib/XML/SAX/ParserDetails.ini). So if you install XML::SAX::Expat, your script will immediately make use of it.