#!/usr/bin/perl

{
	package AssignOnce;

	use strict;
	use warnings;

	use Carp ();
	use Tie::Hash;
	use parent -norequire => 'Tie::StdHash'; # why, oh why, is Tie::StdHash hidden in Tie::Hash?

	sub STORE
	{
		my ($self,$key,$value)=@_;
		if (exists $self->{$key}) {
			Carp::carp("Overwriting hash key '$key', old value: '$self->{$key}', new value: '$value'");
		}
		$self->SUPER::STORE($key,$value);
	}
}

use strict;
use warnings;
use Data::Dumper;

our %h;
tie %h,'AssignOnce';
%h=(
	one => 1,
	two => 2,
	one => 3,
);

print Dumper(\%h);

##</code><code>##

Overwriting hash key 'one', old value: '1', new value: '3' at tie.pl line 29
$VAR1 = {
          'one' => 3,
          'two' => 2
        };