Keys are never auto-vivified. They are always created through assignment (incl the implicit assignment of an autovivified hash or array as shown in an earlier post). 
$h{$k} = $v;
@h{$k} = ($v);
%h = ($k, $v);  # Also removes everything else.

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In reply to Re^5: Hashing it out: defined? exists? by ikegami
in thread Hashing it out: defined? exists? by eff_i_g

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