As is often the case in such cases, i'm the person with the strange opinion here. I like what you have done here.

No, that the code itself. There are better ways, like using tested and more feature-complete modules instead of your messy spaghetti code.

What i like is that you took the initiative, looked into how the file format actually works and in the process learned something!

And what's more: I learned something: I could stuff the generation of Excel files into some Template Toolkit file(*), damn the incompatibilities and full speed ahead. If someone uses Excel, they are used to pain and suffering and random error messages popping up anyway.

(*) Which would come in quite handy if i could pull that off. That would mean i get relatively "free" XLSX generation in one of my web tools without having to implement a second data handling module.