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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

sub blackbox { }   # no autovivify

##</code><code>##

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