Win32::OLE but it's a bad idea. You will also need some strongish VBscript-ing skills to go with it since Many Win32::OLE options are not documented and knowing the VBscript equivalent will allow you to find functions that you need. The actual process is to open the Microsoft Excel application, read the file into Excel with your script (Don't manually try and interact with Excel when the script has it open), make your changes, save the file, and then close Excel again.
From a data-safe standpoint there is very little keeping you from overwriting something accidentally using this process. Additionally these scripts are not very portable since they rely heavily on the Excel settings embedded in each computers Excel installation.
The alternative is to export your perl data with a .csv file using the excellent Text::CSV and either import the data to the existing Excel manually (Data/From Text/FileName.csv) or do the same with a Macro or PowerShell.
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