I'm really unclear what you mean by "Doc Interps", and also what you mean by "spreadsheet is using BIFF 5".

I interpret your question as follows:

You're using Spreadsheet::WriteExcel to create Excel format files. Some mobile Excel document display programs are having problems with some of the produced files because the files seem to be in the BIFF 5 format while the programs seem to expect BIFF 8 format.

If you want to produce BIFF 8 format, I guess the easiest way is to automate Microsoft Excel to produce them. See Win32::OLE and lots and lots of examples here, via Super Search.