$Excel->{DisplayAlerts} = 'False';
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... does not do what you think it does. Most likely you want:

$Excel->{DisplayAlerts} = 0;
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Also, if Excel doesn't tell you the reason, it's hard to find out the reason yourself. You can try to be more defensive:

my $Book = $Excel->Workbooks->Open("$in_file_dgb/haha.xls")
    or die "Couldn't open '$in_file_dgb/haha.xls' via Excel.";
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Also consider using Spreadsheet::ParseExcel to see whether the Excel file actually is readable.