examples/references/autovivification.pl
#!/usr/bin/perl
use strict;
use warnings;

use Data::Dumper;

my $data;
$data->{Foo}{Age} = 23;

print Dumper $data;

if ($data->{Bar}{Age} > 18) {
    print "Your are too old\n";
}

print Dumper $data;


delete $data->{Moo}{Age};

print Dumper $data;

Output:

examples/references/autovivification.out
$VAR1 = {
          'Foo' => {
                     'Age' => 23
                   }
        };
$VAR1 = {
          'Bar' => {},
          'Foo' => {
                     'Age' => 23
                   }
        };
$VAR1 = {
          'Bar' => {},
          'Moo' => {},
          'Foo' => {
                     'Age' => 23
                   }
        };