my %b;
foreach (@a) { $b{$_} = 1 if defined $_ }

##</code><code>##

use Benchmark ':all';

my @a;
push @a, rand(500) for (0..1000);  #build array
$a[int(rand(1000))] = undef;       #delete an element (sort of)
$a[int(rand(1000))] = undef;       #delete an element (sort of)

cmpthese( 1_000, {
	'map'     => sub { my %b = map{ $_ => 1 } @a },
	'foreach' => sub { my %b; foreach(@a) { $b{$_} = 1 if defined $_ } },
});
__END__
         Rate     map foreach
map     374/s      --    -44%
foreach 673/s     80%      --