Keep in mind that even checking for the presence of a hash key that does not exist will auto-vivify it into existence.

Almost :)
Perl will auto-vivify the hashes/hashkeys *necessary* to evaluate an expression.

This will clarify the differences:

perl -MData::Dumper -we 'print Dumper $h1 unless $h1->{a}->{b}->{c}'
$VAR1 = {
          'a' => {
                   'b' => {}
                 }
        };
[download]

In order to test for $h1->{a}->{b}->{c} there must exist a hash for 'c' to be a key in, so Perl auto-vivifies the data structure up to that point. But notice that 'b' points to an empty hash; 'c' is not a key in it! Perl sees that 'c' is not in {'b'}, and that is all that it needs to evaluate the unless.

perl -MData::Dumper -we 'print Dumper $h1 for $h1->{a}->{b}->{c}'
$VAR1 = {
          'a' => {
                   'b' => {
                            'c' => undef
                          }
                 }
        };
[download]

Here, we asked Perl to actually *obtain* the value stored in 'c', so Perl went one step further to create the hash key 'c', which holds 'undef', which it returns to the for statement.

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