JSON::XS requires JSON, so cpan is trying to load JSON to make sure you have it. You have configured JSON to use JSON::XS which isn't installed, so JSON doesn't load.

The solution is to stop telling JSON to use JSON::XS until you've installed JSON::XS. The following installed JSON::XS, after which you'll be fine: 

PERL_JSON_BACKEND= cpan JSON::XS

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In reply to Re^3: JSON 2.5 2.90 by ikegami
in thread JSON 2.5 2.90 by pno

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