What about tie?

#!/usr/bin/perl

{
    package AssignOnce;

    use strict;
    use warnings;

    use Carp ();
    use Tie::Hash;
    use parent -norequire => 'Tie::StdHash'; # why, oh why, is Tie::St
+dHash 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);
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Result:

Overwriting hash key 'one', old value: '1', new value: '3' at tie.pl l
+ine 29
$VAR1 = {
          'one' => 3,
          'two' => 2
        };
[download]

Alexander