@saw{@in} = undef;

##</code><code>##

@saw{@in} = (1) x @in;

##</code><code>##

@saw{@in} = ();

##</code><code>##

  DB<1> use Devel::Size 'total_size'
  DB<2> @x{1..1000}=()  
  DB<3> @y{1..1000}=(1) x 1000
  DB<4> p total_size(\%x)
41049
  DB<5> p total_size(\%y)
45049

##</code><code>##

if ( exists $some_hash{ foo } ) {
  launch_the_missiles();
}

##</code><code>##

$x = $some_hash{ maybe_foo() }{ bar };

##</code><code>##

my @no_repeats = do { my %h; grep !$h{$_}++, @has_repeats };