I've also found that even though Perl can handle forward slashes as path delimiters, Excel can't. For example:

# ... other code before this ...
if (-e "c:/myfiles/test.xls")
{
  # We get here just fine, but...
  $Book = $Excel->Workbooks->Open("c:/myfiles/test.xls")
    or die "Hork!\n";
  # Dies
}
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