You are pretty close. Apparently, the "official" name of this encoding isn't MacRoman, but "macintosh": https://www.iana.org/assignments/character-sets/character-sets.xhtml. It looks like XML::LibXML, or more likely the underlying C libraries, don't recognize the encoding as "MacRoman".

If I just change the next-to-last line to:

...then I get a properly MacRoman encoded file starting with <?xml version="1.0" encoding="macintosh"?>.

Edited to add: You might also consider just deleting that line, unless you really, really need your XML file to be encoded in that rather uncommon MacRoman encoding. UTF-8 is the standard encoding for XML and is understood by all XML processors.