Your results do not surprise me. I personally think of these two as equivalent:

$hash{foo} ||= keys %hash;
[download]

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

At least, the results agree: the hash element exists (hence, it is counted in keys), before the assignment is called. My rule of thumb is: if you use a hash element as a R/W L-value, then it'll autovivify. It'll even autovivify in the for loop, if the loop body is empty:

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

I was surprised to see an exception when using a hash element as a sub parameter:

sub blackbox { ... }
blackbox($hash{foo});
[download]

may or may not autovivify $hash{foo}, depending on what blackbox() does. For example:

sub blackbox { }   # no autovivify
[download]

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

Tested with perl 5.8.7