This should explain it.

say( 
    '$count = () = ( wantarray() ) gives ',
    ( $count = () = ( wantarray() ) ) 
    ? "List" : "Scalar", 
    " context to wantarray()." 
);


say(
    '$count = ( wantarray() ) gives ',
    ( $count = ( wantarray() ) ) 
    ? "List" : "Scalar", 
    " context to wantarray()." 
);
[download]

The output:

$count = () = ( wantarray() ) gives List context to wantarray().
$count = ( wantarray() ) gives Scalar context to wantarray().
[download]