Hash:

123->abc
456->abc
456->xyz
789->abc


##</code><code>##

use strict;
use warnings;
use 5.010;

use Data::Dumper;

my %hash = (
    123 => 'abc',
    456 => 'abc',
    456 => 'xyz',
    789 => 'abc',
);

print Dumper(\%hash);

--output:--
$VAR1 = {
          '456' => 'xyz',
          '123' => 'abc',
          '789' => 'abc'
        };


##</code><code>##

Hash:

123->abc
456->abc
456->xyz
789->abc