Unless there's something going on that I don't understand (in which case I'd love to know), it won't work as you describe. Consider
# get a new workbook $book = $ex->Workbooks->Add;
This creates a new book, i.e. a new file, in the Excel instance. Then, when the code saves test.xls, it's only that book that gets saved. No sheets are added. $sheet is created by the line
$sheet = $book->Worksheets(1);
Again, this is in $book, not in anything that might have been open in the same instance. This can't fail, as the creation of $book will have the default number of sheets from SheetsInNewWorkbook, which can never be less than 1.
Now, I accept that the code given,
# save and exit $book->SaveAs( 'test.xls' ); undef $book; undef $ex;
doesn't actually close the instance of Excel, but merely undefines it in Perl. But this thread is, to a large extent, about closing instances of Excel, so my fear is undiminished.
If you don't have any instances of Excel open, there's no point even trying to take control of an open instance. And if you do, we return to the dangers on which we agree. If the code creates its own instance without trying to take control of an existing instance, you are safe either way.