A hash can't have a key without a value (that's why I refer to key/value pairs). If you reference $h{x} in some fashion (except as a target for exists) then a x/undef pair is created (autovivified) for %h if it didn't exist already. The undef returned by $h{x} is the value associated with the key 'x'.

"Adding" an undef value to %h when $h{x} is used is exactly what autovivification does.

Autovivification (in the case of a hash) is when a key is added to a hash where previously that key wasn't there. Just using $h{x}, as in $val = $h{x} does not add a new key to the hash. It simply assigns the default undef value to $val. Referencing through a key does autovivify the all but the last key:

#!/usr/bin/perl
my %h;
my $val = $h{x}{y};
use Data::Dumper;
print Dumper \%h;
[download]

Output:

$VAR1 = {
          'x' => {}
        };
[download]

Apparently, not only does $h{x} not autovivify $h{x}, but even %{$h{x}} does not autovivify $h{x} (you have to turn off strict and -w first):

#!/usr/bin/perl
my %h;
%{$h{x}};
use Data::Dumper;
print Dumper \%h;
[download]

Output:

$VAR1 = {};
[download]

But, using keys() causes %{$h[x}} to autovivify $h{x}.

#!/usr/bin/perl -w 
use strict;
my %h;
keys %{$h{x}};
use Data::Dumper;
print Dumper \%h;
[download]

Output:

$VAR1 = {
          'x' => {}
        };
[download]