I get the opposite results. The uncommented loop works and the commented loop gives a warning.

The warning occurs because SheetCount is the index of the last worksheets +1. So I guess this is what you want (although I prefer the other method):

foreach (0 .. $book->{SheetCount} -1) {
    $sheet = $book->{Worksheet}[$_];
    print $sheet->{Name},"\n";
}
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--
John.