Wild guess: if you built your perl with perlbrew before upgrading the OS that (or some of the config baked therein) may be the source of the older version. Try reinstalling your perl (although if that's the problem you may need to recompile all the XS modules as well to ensure they match the new binary).

I thought for sure you were on to something. I installed latest version of perl with perlbrew and migrated all the modules over and I switched to latest version. Still getting this error, however, with "cpanm Config::Augeas":

Building Config-Augeas
cc -I/Users/me/perl5/perlbrew/perls/perl-5.32.1/lib/5.32.1/darwin-2lev
+el/CORE -DVERSION="1.000" -DXS_VERSION="1.000" -I/usr/local/Cellar/au
+geas/1.12.0/include -Wall -Wformat -Werror=format-security -c -fno-co
+mmon -DPERL_DARWIN -mmacosx-version-min=11.2 -fno-strict-aliasing -pi
+pe -fstack-protector-strong -I/usr/local/include -DPERL_USE_SAFE_PUTE
+NV -O3 -o lib/Config/Augeas.o lib/Config/Augeas.c
ExtUtils::Mkbootstrap::Mkbootstrap('blib/arch/auto/Config/Augeas/Augea
+s.bs')
cc -mmacosx-version-min=11.2 -bundle -undefined dynamic_lookup -L/usr/
+local/lib -fstack-protector-strong -o blib/arch/auto/Config/Augeas/Au
+geas.bundle lib/Config/Augeas.o -L/usr/local/Cellar/augeas/1.12.0/lib
+ -laugeas
Can't call method "get" on an undefined value at /Users/stevedondley/.
+cpanm/work/1615424668.56822/Config-Augeas-1.000/blib/lib/Config/Augea
+s.pm line 227.
[download]

# Looks like your test exited with 255 just after 2.

$PM = "Perl Monk's";
$MCF = "Most Clueless Friar Abbot Bishop Pontiff Deacon Curate Priest Vicar";
$nysus = $PM . ' ' . $MCF;
Click here if you love Perl Monks

You can influence CFLAGS and LDFLAGS via the Module::Build constructor. It is documented here http://perl.mines-albi.fr/perl5.6.1/site_perl/5.6.1/Module/Build.html (but not in CPAN, wrong oops: it's here: https://metacpan.org/pod/distribution/Module-Build/lib/Module/Build/API.pod)

 my build = Module::Build->new(
     extra_compiler_flags => '... -mmacosx-version-min=11.0',
     extra_linker_flags   => '... -mmacosx-version-min=11.0',
 );
[download]

The problem is that Augeas' Build file has already some flags specified there. So, you could try to modify them with something like this (untested, just a rough sketch):

my $extra_cflags = $aug_cflags . ' ' . $libxml2_cflags . ' -Wall -Wfor
+mat -Werror=format-security';
my $extra_ldflags = $aug_libs . ' ' . $libxml2_cflags;
$extra_cflags =~ s/mmacosx-version-min=10\.13/mmacosx-version-min=11.0
+/g;
$extra_ldflags =~ s/mmacosx-version-min=10\.13/mmacosx-version-min=11.
+0/g;
print "Modified to $extra_cflags\n" ;
print "Modified to $extra_ldflags\n" ;


# insert the above before this:
print "Using $aug_libs and $aug_cflags to compile (Augeas version $aug
+_version)\n" ;
print "Using $libxml2_libs and $libxml2_cflags to compile (Augeas vers
+ion $aug_version)\n" ;
[download]

And then in the constructor

my $build = Module::Build->new
  (
 ...
  extra_compiler_flags => $extra_cflags,
   extra_linker_flags   => $extra_ldflags,
 ...
);
[download]

BTW, the option dynamic_config => 0 will create an intermediate file Build.PL which can be modified as per my 3rd suggestion. Currently for Augeas this is set to 1

Fletch's suggestion is also worth checking, perhaps with this:

perl -MConfig -e 'print $_.": ".$Config{$_}."\n" for keys %Config;'
[download]

bw, bliako

Thanks for your detailed help. I took fletch's advice and reinstalled a fresh new version of Perl compiled against my current OS. As far as I can tell, the module compiles. However, it fails its tests. (Please see my comment above.)

My instinct tells me that the module is having trouble finding Augues. I know Augeas is installed and works on my system because I can do this little tutorial found here: https://augeas.net/tour.html

I tried taking a look at the source code of the module to see if I could spot the problem. But all I found was this call to an init function and I'm not sure how it works. I couldn't find a sub called "init" anywhere else in the distribution. Maybe it's an XS thing?

Any other ideas on what I can try to get this working?

$PM = "Perl Monk's";
$MCF = "Most Clueless Friar Abbot Bishop Pontiff Deacon Curate Priest Vicar";
$nysus = $PM . ' ' . $MCF;
Click here if you love Perl Monks

it looks like https://metacpan.org/source/RAPHINK/Config-Augeas-1.000/lib%2FConfig%2FAugeas.pm#L142 calls https://metacpan.org/source/RAPHINK/Config-Augeas-1.000/lib%2FConfig%2FAugeas.xs#L86. It looks like it fails to init but none-the-less that undef return value is stored in the $self, and when get() is called it is on an undef ref. But I have no idea what's going on further down inside XS. If you manage to insert here https://metacpan.org/source/RAPHINK/Config-Augeas-1.000/lib%2FConfig%2FAugeas.pm#L141 a debug message showing $root and $loadpath you may be able to get a hint at least where does it look for libraries. That means something like: 1) run the cpanm command which will fail, 2) chdir to the cpanm work dir mentioned in the error message, 3) edit Config::Augeaus within this path with diag messages, 4) Somehow try to initiate the build process again MANUALLY (not via cpanm), usually with a make, or perl Makefile.PL or ./Build but beware what I mentioned earlier on how to force it to create a Build file.

FWIW I managed to macport install augeas and then cpanm Config::Augeas with no problems, on OSX 10.13