I think the gcc version may have been a red herring

Quite possibly, but it seems that there's something about these old RHEL systems and gcc compilers that's triggering a bug - either a bug in List-MoreUtilsXS-0.428 or a bug in the way the handshake compatibility is being established.

I would suggest building perl with that handshake mechanism disabled - just to see if there is anything that is actually incompatible about XS.c. (Perhaps there's absolutely nothing wrong with XS.c.)
However, I don't know how to build such a perl without hacking at util.c (in the top level directory of the perl source).
If you think it's worth the effort, try changing (in util.c): 
noperl_die("%s: loadable library and perl binaries are mismatched"

[download]
to a non-fatal: 
Perl_warn(aTHX_ "%s: loadable library and perl binaries are mismatched
+"

[download]
or even just: 
printf("%s: loadable library and perl binaries are mismatched"

[download]
And then rebuild perl and see how List-MoreUtils-XS-0.428 fares.
(I haven't tested either of those suggested hacks.)

Could you also post the config.log of the L::MU::XS build that is generated by the perl Makefile.PL step. (You'll find that config.log in the same directory as the Makefile.PL).
It might provide some clue as to the cause of the problem.
And it might be worth filing a bug report about this at https://rt.cpan.org/Public/Dist/Display.html?Name=List-MoreUtils-XS in case the author has some ideas.

Thanks for updating us !!

Cheers,
Rob

In reply to Re^18: XS.c: loadable library and perl binaries are mismatched (got handshake key 0xc100000, needed 0xc180000) by syphilis
in thread XS.c: loadable library and perl binaries are mismatched (got handshake key 0xc100000, needed 0xc180000) by rgren925

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