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