Here's a little snip I put together messing around with autovivification. Perhaps some monks that feel queasy about this topic may find it useful.

For a more in depth tutorial on the topic The Bad, the Ugly, and the Good of autovivification may be of interest. Cheers :)

use strict;
use warnings;
use Test::More qw(no_plan);
use Data::Dumper;

my %hash = (a => 'a', b => 'b');

ok( ! exists($hash{c}), "hash -- c doesn't exist");
ok( ! exists($hash{c}), "hash -- c still doesn't exist");
ok( ! defined($hash{c}), "hash -- c not defined either");


my $hashref = { %hash };

ok( ! exists($hashref->{c}), "hashref -- c doesn't exist");
ok( ! exists($hashref->{c}), "hashref -- c still doesn't exist");


ok( ! exists( $hashref->{c}->{d} ), "hashref, deeper -- c->{d} doesn't
+ exist");
ok( ! exists( $hashref->{c}->{d} ), "hashref -- c->{d} still doesn't e
+xist");
ok( ! exists($hashref->{c}), "hashref -- c still doesn't exist"); #fai
+ls. c does exist.
ok( ! defined($hashref->{c}), "hashref -- c still doesn't exist"); #fa
+ils. c is an empty hashfref, which is something other than undef.
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