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;
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Output:

$VAR1 = {
          'x' => {}
        };
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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;
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Output:

$VAR1 = {};
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But, using keys() causes %{$h[x}} to autovivify $h{x}.

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

$VAR1 = {
          'x' => {}
        };
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