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.

In reply to The Two Auto-Vivifies by Util
in thread re-using a hash reference (was: Mar) by Anonymous Monk

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