my %DATA = (3 => {});

put_element($DATA{3}, ...);

##</code><code>##

my %DATA = (3 => {});

put_element($DATA{42}, ...);

##</code><code>##

sub test1 {
  my $val = shift;
  $val = 5;
}

sub test2 {
  $_[0] = 5;
}

my ($a, $b) = (3, 3);

test1($a); # $a continues to be 3.
test2($b); # $b is 5 now.