try this, the () forces list context and then after that, $y can be the number of elements.

use warnings;
use strict;

my $y = () = ("a", "b");
print "$y\n";

__END__ 
Prints:
2
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Update: When writing this post, I knew that this "secret" operator had a cute name but I couldn't remember it. I found it: Goatse also called the "Saturn" operator, =()=.