$ cat autovivify.pl 
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dump qw(dump);

my %h = (A=>0);

print "Foo" if defined $h{Apple}{Banana}{Seahorse};

$h{Apple}{Banana}{Seahorse}=undef;

print "Bar" if defined $h{Apple}{Banana}{Seahorse};

print dump(\%h);

$ perl autovivify.pl
{ A => 0, Apple => { Banana => { Seahorse => undef } } }