The problem has nothing to do with how the spreadsheet is generated. When using your code

use strict;
use warnings;
use 5.010;
#use Spreadsheet::WriteExcel;
use Win32::OLE qw(in with);
use Win32::OLE::Const 'Microsoft Excel';
$Win32::OLE::Warn=3;

my $Excel=Win32::OLE->GetActiveObject('Excel.Application')||
    Win32::OLE->new('Excel.Application', 'Quit');

my $book=$Excel->Workbooks->Open("./test.xls");
my $sheet=$book->Worksheets(1);
[download]

I get the same error on a spreadsheet I created myself with Excel 2003. Using the slightly modified code

use strict;
use warnings;
use 5.010;
#use Spreadsheet::WriteExcel;
use Win32::OLE qw(in with);
use Win32::OLE::Const 'Microsoft Excel';
$Win32::OLE::Warn=3;

my $Excel=Win32::OLE->GetActiveObject('Excel.Application')||
    Win32::OLE->new('Excel.Application', 'Quit');

my $book=$Excel->Workbooks->Open("c:\\data\\temp\\test.xls");
my $sheet=$book->Worksheets(1);
[download]

it opens the workbook just fine.