What I think is happening is the following:

Could be :)

perldoc Spreadsheet::ParseExcel |grep -i formula

This module cannot read the values of formulas from files created with Spreadsheet::WriteExcel unless the user specified the values when creating the file (which is generally not the case). The reason for this is that Spreadsheet::WriteExcel writes the formula but not the formula result since it isn't in a position to calculate arbitrary Excel formulas without access to Excel's formula engine.