my @x = map { if ($_ > 0) { $_*2 } } (-2 .. 2);
use Data::Dumper;  print Dumper \@x;

__END__
$VAR1 = [
          '',
          '',
          '',
          2,
          4
        ];

##</code><code>##

my @x = map { if ($_ > 0) { $_*2 } else { () } } (-2 .. 2);
# or: my @x = map { $_ > 0 ? $_*2 : () } (-2 .. 2);
use Data::Dumper;  print Dumper \@x;

__END__
$VAR1 = [
          2,
          4
        ];