It doesn't matter how big the file is. Excel has a row limit (65536), and the OP is hitting that limit. Try:

#!/usr/bin/perl

use Spreadsheet::WriteExcel;

my $xls = Spreadsheet::WriteExcel->new('tmp.xls')
or die "Blah: $^E";

my $ws = $xls->add_worksheet();

my $row = [1..5];

for (0..70000) {
  $ws->write_row($_, 0, $row);
}
$xls->close;
[download]

No errors, but the spreadsheet will only have 65536 rows (maybe it depends on the version of Excel? Hey, just like we'd never need more than 640K of memory, we'd never need more than 65536 rows, would we? :-). Your script would have been a good starting point for the OP though.

BTW, recent versions of S::WE automatically use the "Big" version when needed (so same results if you use 'Big' above).