(Almost?) every non-alpha printable character is used as a global variable for something. In this case, $( is defined in perlvar as $REAL_GROUP_ID (Whatever that is:).

But every global scalar, is just one part of a 'glob', which also has 6 (or 7?) 'slots' (*).

You are familiar with $_ and @_, well there is also a %_ which doesn't see much use outside golf. So it is with all other globals, the are $|, @| & %|, only the first of which has a predefined use, but the others exist and are usable as with any other general purpose global.

(*). I forget exactly what they all are, but broquaint did a very good tutorial on them somewhere. See that for the full SP. Try supersearch for tutorial glob and author broquaint.