First of all: it's OK. If nothing else, in either 2017 (first copyright date) or 2020 (first CPAN), PDL::IO::STL didn't exist.

Also, that doesn't (yet) support normals in any useful way; it drops them on input, and writes out 0s on output. Does anything actually rely on them being correct in the file?

Finally, it seems to me that CAD, and 3D stuff in general, is really the killer app for PDL. The array-programming stuff makes things quick because fast C code, and also very easy to write. But especially because all the 3D maths stuff (including cross product) are just there already, together with any linear algebra stuff you might need to do. You can write lots of object-y Perl, with methods, or write 3 lines of PDL code. Truly, more than one way to do it ;-)