Run latest enc2xs/makemaker then see whats what

Yeah - the OP could give that a go.
What does it involve exactly ? That is, what's the exact command that gets run, and from which directory does one need to run it ?

The oldest Windows perl I have is perl-5.8.8 (enc2xs-2.01) and it's building Encode-IBM-0.11 just fine apart from a million or so warnings about:

attention : initialization discards 'const' qualifier from pointer tar
+get type [enabled by default]"
[download]

But the build succeeds, and the test suite passes ... and no bizarre warnings produced during the 'perl Makefile.PL' step.

The oldest Strawberry Perl I have is perl-5.12.2 (enc2xs-2.07) which also builds Encode-IBM-0.11 just fine (and no warnings).
Strawberry Perl 5.30.0 has enc2xs-2.21.
According to the note at the beginning of the Makefile.PL (which I hadn't noticed), it was generated using enc2xs-2.05.

Cheers,
Rob