After running this myself, the only outcome that I didn't expect, was that exists $hash{d} returns false after print $hash{d}. I had expected the print to autovivify the d key to contain undef.

The thing to remember is that it is not hash values that autovivify -- it is anonymous hash and array references that autovivify. For example, given an undef scalar, you can autovivify the anonymous hash just by coding as if the scalar contained a hash reference:

use strict;
use warnings;

local $\="\n";
my $hr;

print '$hr ',      defined $hr     ? 'defined' : 'undefined';

print '$hr->{a} ', exists $hr->{a} ? 'exists'  : 'doesn\'t exist';

print '$hr ',      defined $hr     ? 'defined' : 'undefined';

print '$hr is ',   $hr;
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Prints:

$hr undefined
$hr->{a} doesn't exist
$hr defined
$hr is HASH(0x3d51c0)
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