$|++;
my $href = { one => 1, two => 2 };

print "\nBefore:\n";
print Dumper($href);

set_element($href);
print "\nset_element:\n";
print Dumper($href);

assign_new_href($href);
print "\nAttempt to assign new hashref:\n";
print Dumper($href);

##########
sub assign_new_href {
    my $href = shift;
    $href = { new_key => 4 } ;
}

##########
sub set_element {
    my $href = shift;
    $href->{baz} = 4;
}

##</code><code>##

Before:
$VAR1 = {
          'one' => 1,
          'two' => 2
        };

set_element:
$VAR1 = {
          'baz' => 4,
          'one' => 1,
          'two' => 2
        };

Attempt to assign new hashref:
$VAR1 = {
          'baz' => 4,
          'one' => 1,
          'two' => 2
        };