%ENV is meant to expose ENV to Perl. In ENV You just cannot have datastructures, only strings. Even if you can add a datastructure to the Perl %ENV hash i think is not a reliable behaviour. %ENV has also a lot of magic underlying: think about case of the keys. 
perl -MData::Dumper -e "$ENV{MyExtraConfig} = { foo=>{Bar => 'Blaz'}};
+print Dumper (\%ENV);system qq(set)"

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UPDATE: in the docs is said "As of v5.18.0, both keys and values stored in %ENV are stringified."
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In reply to Re: Cannot write data struct to %ENV by Discipulus
in thread Cannot write data struct to %ENV by exilepanda

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