You have figured out some of this already but just to give you a definitive answer:

Third party/add-in functions aren't supported by Spreadsheet::WriteExcel and there is no easy way to add them

Third party/add-in functions are supported by Excel::Writer::XLSX which requires perl 5.10.

There is a version of Excel::Writer::XLSX on GitHub that supports perl 5.8.2 which you might be able to backport to 5.6.0 with an reasonable amount of work.

--
John.