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$hash{foo} ||= keys %hash;

for($hash{foo}) { $_ ||= keys %hash; }

for($hash{foo}) { }   # results in { 'foo' => undef }

sub blackbox { ... }
blackbox($hash{foo});

sub blackbox { }   # no autovivify

sub blackbox { $_[0] = 123 }   # results in { 'foo' => 123 }