I've recently begun using Spreadsheet::WriteExcel. If you're getting a new worksheet each time a sub is called, you may have

$ws = $workbook -> add_worksheet();
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In the sub; this will create a new worksheet on each call, numbering them sequentially (1, 2, .... n). You could pass $ws to the sub, and manage worksheet creation in the calling routine (my preference) or surround the add_worksheet call with some logic so it's not invoked after the first invocation.