Your results do not surprise me. I personally think of these two as equivalent: 
$hash{foo} ||= keys %hash;

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

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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 }

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

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

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may or may not autovivify $hash{foo}, depending on what blackbox() does. For example: 
sub blackbox { }   # no autovivify

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

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Tested with perl 5.8.7

In reply to Re: Are "$hash{$_} ||= 1 + keys %hash" and variants well defined or not? by bart
in thread Are "$hash{$_} ||= 1 + keys %hash" and variants well defined or not? by demerphq

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