Okay, with the following code, I was able to make an exploding pie chart.

There are things here that I'm not happy with. Ideally, the ...::Pie::_write_ser should mostly just run the ::Chart::_write_ser but somehow insert the call to _write_explosion; I didn't know how to do that. It may be that the "right" answer is for ::Chart::_write_ser to include the call to _write_explosion itself, rather than having the ::Pie do it... Maybe then having ::Chart::_write_explosion be empty but ::Chart::Pie::_write_explosion be defined as in my code. But for the v1.00 and v1.09 of Excel::Writer:::XLSX, the code I shared worked.

Again, really, it should be incorporated into the distribution with a feature request -- if you do create such a feature request, you have my permission to point the author to this post, and the author may include the code I presented here with whatever fixes or adaptations are necessary to make it work with the distro as a whole.