the expression %{ $a } isn't in lvalue context.

Yes, it is.

$ perl -E'sub f { $_[0] = 'abc' }  f($x); say $x'
abc

$ perl -E'sub f { $_[1] = 'abc' }  $h{a}=undef; f(%h); say $h{a}'
abc
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Perl arguments are always passed by reference.

Also autovivification doesn't appear only in lvalue context

I didn't say otherwise.