use strict;
use Tie::Hash;
use Carp;

package AccessControl;
@AccessControl::ISA = 'Tie::StdHash';

sub allowVivification { $_[0]->{__novivify} = not $_[1] }

sub TIEHASH { bless { __novivify => 0 }, $_[0] }

sub STORE
{
 my ( $self, $key, $value ) = @_;
 if ( $self->{__novivify} )
 {
  unless ( exists( $self->{$key} ) )
  {
   Carp::carp("Autovivification of key $key not allowed");
   return;
  }
 }
 $self->{$key} = $value;
}

sub FIRSTKEY
{
 my $a   = scalar keys %{ $_[0] }; # reset each iterator
 my $key = each %{ $_[0] };
 $key eq '__novivify' ? each %{ $_[0] } : $key;
}

sub NEXTKEY
{
 my $key = each %{ $_[0] };
 $key eq '__novivify' ? each %{ $_[0] } : $key;
}

package main;

my %hash;
my $accessControl = tie %hash, 'AccessControl';

$hash{abc} = 'def';    # OK
$accessControl->allowVivification(0);
$hash{ghi} = 'xxx';    # carps
$accessControl->allowVivification(1);
$hash{jkl} = 'yyy';    # OK
print join ( ' ', keys(%hash) ), "\n";   # abc jkl