Fixed this myself (I was 99% there!). Using the macro recorder within Excel confirmed the required function calls as follows:
ChDir "C:\temp"
ActiveWorkbook.SaveAsXMLData Filename:="C:\temp\master.xml", Map:= _
ActiveWorkbook.XmlMaps("MDBAC_Map")
The line of code for exporting the XML map is wrong. Changed the above code as follows and the script works fine:
my $objMapToExport = $workbook->XmlMaps("MDBAC_Map");