Is the Perl Goatse 'Secret Operator' efficient?

The "goatse operator" or the =()= idiom in Perl causes an expression to be evaluated in list context.

An example is:

my $str = "5 and 4 and a 3 and 2 1 BLAST OFF!!!";
my $count =()= $str =~ /\d/g; # 5 matches...
print "There are $count numbers in your countdown...\n\n";

As I interprete the use, this is what happens:

1. $str =~ /\d/g matches all the digits. The g switch and list context produces a list of those matches. Let this be the "List Producer" example, and in Perl this could be many things.
2. the =()= causes an assignment to an empty list, so all the actual matches are copied to an empty list.
3. The assignment in scalar context to $count of the list produced in 2. gives the count of the list or the result of 5.
4. The reference count of the empty list =()= goes to zero after the scalar assignment. The copy of the list elements is then deleted by Perl.

The questions on efficiency are these:

1. Am I wrong in how I am parsing this?
2. If you have some List Producer and all you are interested in is the count, is there a more efficient way to do this?

It works great with this trivial list, but what if the list was hundreds of thousands of matches? With this method you are producing a full copy of every match then deleting it just to count them.