"If anyone has other information re PDF::API2 vs. PDF::Builder, please add comments."

Thanks to a private message from pryrt: "PDF::Builder::Docs - additional documentation for Builder module". (Pity you can't give a /msg a ++.)

In particular, the History section which describes PDF::API2 → PDF::Builder: similar to what I guessed, but it's a lot more involved.

"... repeated the test ... replacing API2 with Builder ..."

Apparently, not just a bit of luck. From the same section: 'At least initially, any program written based on PDF::API2 should be convertible to PDF::Builder simply by changing "API2" anywhere it occurs to "Builder".' — so, for anyone wishing to upgrade applications from PDF::API2 to PDF::Builder, it's possibly as easy as a simple global change: s/API2/Builder/g.