sub f {
    my ($r) = @_;
    $r->{level3} = 1;
}

my $rh;
f($rh->{level1}{level2});

##</code><code>##

rh->{level1}{level2}){level3} = 1;

##</code><code>##

f(\$rh->{level1}{level2}->%*);