Here is something interesting. After trying several CAM::PDF external methods to see if anything has an effect on ->cleanoutput, I re-read the documentation and found a reference to ->cleanse, which is listed under "Routines that should not be called," in the context of throwing away unused font data.

Well, I added it to my script anyway and now the combined document saves without error.

So, I thought this is useful to pass along. Thanks for all the replies. I think this site is exceptionally helpful. Regards.