Win32::GetLastError will only return a meaningfull value if called immediately after the system call. It most likely doesn't return Excel-specific information, so likely you're out of luck with your strategy of trying to find out something more specific about the error condition.

There seems to be no way to convince Excel to save the file unconditionally with the ->SaveAs method, overwriting what is potentially there, so I think you'll have to use

$excel->{DisplayAlerts} = 0;
$excel->Workbooks(1)->SaveAs( $sp_full );
[download]

The documentation doesn't say much on using the return value of ->SaveAs, so you might have to guess whether it worked or not.