$master_ref = ( $master_ref, return_hash_ref() ); # line 18

##</code><code>##

my $returned = return_hash_ref();
@{$master_ref}{keys %$returned} = values %$returned;

# OR
my $returned = return_hash_ref();
$master_ref->{$_} = $returned->{$_} for keys %$returned;

##</code><code>##

$master_ref = { %$master_ref, %{return_hash_ref()} };