Geek note: I found the incompatibility that made a perl build on one Debian Sid box spew errors about errno, and not the other. It was in the libbind-dev package. When it is installed, perl automatically links with -lbind and that library is not compatible with the new glibc, thus causing the tiresome error.
Now there’s a week of debugging I’d like back. HURF! But done now.