I'm guessing from your code that you're using Spreadsheet::WriteExcel. The only way I can see that error happening is if the sheet name snuck into a formula. What I would suggest is to check for it explicitly, something like this:

$value = $storage_book->{$sheet}->{$row}->{$col};
if ($value =~ /sales!/i)
{
    print "The culprit cell is at $row:$col\n";
}
$dest_sheet->write($row, $col, $value);
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This is just a SWAG. If that's not the problem, please post a reply with more details: some example data, the modules you're using, and a wider window around line 265.