Woooowww..This has to be fast.

Don't guess. Test!

Firstly that it does the same as yours does.

And when you are sure it does, then time both with the same data.

can you please explain what this statement does? push @{ exists $a{ $_ } ? $d : $c }, $_ for keys %b;

1. It iterates over the unique values in the B array,
2. and if that value exists in the A array, it exists in both, so it pushes it to the D (intersection) array.
3. otherwise, it only exists in the B array, so it pushes it to the (B only) C array.

and how can u have Hash %a and array ref @$a?

There is no conflict. You can have $a, @a, & %a all in the same scope.

But, rename them to whatever suites your personal preference.