use Data::Dumper;

sub foo {
    $_[0] = undef;
}

sub bar {};

my @a;

@a = ();
foo( $a[0] );

print Dumper \@a;

@a = ();
bar( $a[0] );

print Dumper \@a;
[download]

This prints:

$VAR1 = [
          undef
        ];
$VAR1 = [];
[download]

As I read the paragraph you quoted, some earlier versions of perl would have produced identical output in both cases.

-guinex