$a[ $_ * 20 ] = $_ for 1 .. 1e5;;

$found = 0; say time; defined( $a[ $_ ] ) and ++$found for 1 .. 20e5; say time; say $found;;
1359141040.60138
1359141041.00958
100000

$h{ $_ * 20 } = $_ for 1 .. 1e5;;

$found = 0; say time; exists( $h{ $_ } ) and ++$found for 1 .. 20e5; say time; say $found;;
1359141122.04252
1359141123.06596
100000