Line 18 currently looks like this:

$master_ref = ( $master_ref, return_hash_ref() ); # line 18
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Change it to one of the two following options:

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

# OR
my $returned = return_hash_ref();
$master_ref->{$_} = $returned->{$_} for keys %$returned;
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If you don't mind changing the underlying reference held by $master_ref (in other words, if nobody else is relying on $master_ref pointing to a specific hashref), you could do this:

$master_ref = { %$master_ref, %{return_hash_ref()} };
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But if someone else also holds a reference to the same hash that $master_ref held, that relationship would be broken.