As always, you do this by using Win32::OLE to automate Excel.

The steps are the usual six steps:

1. Start the Excel Macro Recorder
2. Perform the desired action(s) manually, in your case, save the file as XML
3. Stop the Excel Macro Recorder
4. Inspect the generated VB code
5. Check that the generated VB code does what you want
6. Translate the generated VB code to Perl / Win32::OLE invocations

There are many examples on this website which should give you a good start on how to do the translation. It consists mostly of s!\.!->! applied to every line of the code, and you will have to exchange the global properties like "ActiveWorkSheet" by the local values you get from $excel->OpenFile().