In the statement

my $aap = \();
[download]

perl is assigning to $app in scalar context. So the reference operator \ creates a reference for every element in the list, and returns the last element. Since there are no elements, this is a reference to undef. Consider

my @a = (1,2,3);
my $b = 4;
my $aap = \(@a,$b);
print Dumper($aap);
my $aap = \($b,@a);
print Dumper($aap);
__OUTPUT__
$VAR1 = \4;
$VAR1 = [
          1,
          2,
          3
        ];
[download]

It is helpful to think of a list on the RHS as an initializer that can be evaluated in different contexts.