perl -e 'print "$_ = $ENV{$_}\n" for sort keys %ENV;'
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Run that. It will print of all your system environmental variables. Which includes the current users 'My Document' folder.

It should be noted, that if you are using Win32::OLE to create the excel file that simply saying

$Book->SaveAs('file.xls');
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will save to the users My Documents folder. (The OLE object does not default to using your programs current working directory).