Maybe map isn't what you want:

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

Using foreach is probably faster anyhow. FWIW:

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%      --
[download]

Note: with warnings on, the map is even slower, because a warning is generated every time an undefined value is encountered in the array. I turned warnings off after discovering this, and so the numbers above represent a sort of best-case.