Any ideas on its disappearance in v5.26 (and later versions)?

The condition that triggers the warning (in MM_Any.pm) is: 
if ($self->{ABSTRACT} && $self->{ABSTRACT} =~ m![[:cntrl:]]+!) { # war
+ning is given }

[download]
That condition is still there in perl-5.26, and I can't see any change to it.
Perhaps a change in the regex engine was introduced in 5.26.

Are you able to verify that the abstract does, in fact, =~ m![[:cntrl:]]+! in perl-5.20 but not in perl-5.26 and later ?
The question would then be "Did that change in the regex fix a problem, or did it create one ?".
From your description of the ABSTRACT, I'm not sure that it does actually contain any control characters.

Update:
I'm having no luck in reproducing your issue at all (on Windows).
I find that it's the same behaviour from 5.20 onwards. For me, the presence of either chr(10) ("\n") or chr(13) ("\r") always results in a match.
What's the perl -V ?

Cheers,
Rob

In reply to Re^3: 'perl Makefile.PL' warning for v5.20 to v5.24 by syphilis
in thread 'perl Makefile.PL' warning for v5.20 to v5.24 by kcott

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