my $x and my($x) differ only when it's used on the left hand side of an assignment. In that case, when you write my $x = FOO, FOO is evaluated in scalar context, if you say my($x) = FOO, it's evaluated in list context, and the first value in the list is assigned to $x (undef if the list is empty).

A common example is a sub starting with sub { my($x) = @_; meaning that the sub would receive one argument and copy it to $x. In this case, sub { my $x = @_; wouldn't work, as it would store the number of arguments to $x.

This is, however, not specific to my: if you write an assignment like $x = FOO or ($x) = FOO, the same rules apply, only it doesn't create a new lexical variable $x but uses the existing variable.