Thanks Rob, for reply and suggestion. I think Perl v5.18.2 in my macOS is hopelessly broken, at least for what I want to do. There have been other reports of issues with Perls built by Apple, as in:

https://groups.google.com/forum/#!msg/perl.perl5.porters/mKhKYbPqrmQ/2Z7rAsyJOsMJ

Notice Hafen McCormick's reference to a 'diagnostic wrapper', presumably a Perl shared library built from an .xs file that would appear to have shipped with the writer's Perl distro from Apple. That wrapper may be mangling the output from the STR_WITH_LEN macro. I am giving up on Perl from Apple, especially in High Sierra, given the huge number of bugs reported by HS users, particularly the almost totally broken backward support for USB 3.0. For example, I have an On-The-Go external enclosure I use to clone/backup my mac with Carbon Copy Cloner. I can clone to a drive in that enclosure but I can't boot from it. If my internal SSD were to fail, I could move the drive from the enclosure to inside the mac and boot from it while waiting for a replacement SSD. Then, clone to the SSD in the enclosure and reverse the process. Apple has yet to respond to my bug report or to the numerous others from USB 3.0 users whose backups are hosed. No point in shopping for/buying a new enclosure since all with firmware prior to High Sierra are probably broken and their seller's don't know it and/or can't or won't certify it as bootable. But I digress...

I looked at several Perls I have access to and as you reported, all have identical XSUB.h and handy.h headers, at least as they relate to macro XS_VERSION_BOOTCHECK, which means the problem must be something either compiled into my Perl or a shared library.

I installed Homebrew and downloaded Perl 5.26.1 and am in the process of configuring my environment to use it instead of Apple's Perl. But, if it were possible to learn that 1) 'yes', such a 'diagnostic wrapper' does exist, and 2) can it simply be removed to make Apple's Perl behave as expected, I would try that. I hate to give up on a problem without knowing why it's a problem. How can I learn whether my Perl is so configured?

Damn, something just occurred to me. Apple does not want macOS users to install additional bits in Apple protected directories, where Perl is installed, so, deliberately hosing the XSUB build process is one way to do that, and then ignore all bug reports related to it. Even Apple recommends installing Perl with brew. I am not a conspiracy theorist but dealing with Apple has made me very paranoid :-(