Hash:

123->abc
456->abc
456->xyz
789->abc
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But...

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'
        };
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The corollary to "keys can have only one value" is "a hash cannot have duplicate keys". If a hash could look like this:

Hash:

123->abc
456->abc
456->xyz
789->abc
[download]

...how would perl know what value to return when you said, "Give me the value associated with the key '456'?