Greetings, I have this code where tests one and two fail. what does the error in line 18 mean in this instance and why are those hash keys destroyed?

#!/usr/bin/perl

use strict;
use warnings;
use Test::More;
use Data::Dumper;

my $master_ref = {
   one => "first",
   two => "second",
};

sub return_hash_ref {
   return { three => "third", four => "forth" };
};

print "Before ".Dumper( $master_ref );
$master_ref = ( $master_ref, return_hash_ref() ); # line 18
print "After ".Dumper( $master_ref );

ok( $master_ref->{ one   } eq 'first',  'one' );
ok( $master_ref->{ two   } eq 'second', 'two' );
ok( $master_ref->{ three } eq 'third',  'three' );
ok( $master_ref->{ four  } eq 'forth',  'four' );

done_testing;
[download]

Running it:

Useless use of private variable in void context at appendhashref.pl li
+ne 18.
Before $VAR1 = {
          'two' => 'second',
          'one' => 'first'
        };
After $VAR1 = {
          'four' => 'forth',
          'three' => 'third'
        };
Use of uninitialized value in string eq at appendhashref.pl line 21.
not ok 1 - one
#   Failed test 'one'
#   at appendhashref.pl line 21.
Use of uninitialized value in string eq at appendhashref.pl line 22.
not ok 2 - two
#   Failed test 'two'
#   at appendhashref.pl line 22.
ok 3 - three
ok 4 - four
1..4
# Looks like you failed 2 tests of 4.
[download]

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