#!/usr/bin/perl -W
use strict;
use warnings;
use Data::Dumper;
my %hash;
if (defined $hash{a} && defined $hash{a}{b}) {}
print "Case 1 without autovivication:\n" , Dumper \%hash;
if (defined $hash{a}{b}) {}
print "Case 2 with autovivication:\n" , Dumper \%hash;