You'll also need a special case for '( \\( ( three ) ) )'. In this case, there are three sets of parens, because the backslash itself is escaped.

The solutions quickly become ugly, complex, and error-prone, which is why the tried and proven module Text::Balanced is a good idea.

Update: Ok, MUBA's approach handles the escaped-backslash case too.

My point is that if the idea of Perl is to make easy things simple and difficult things possible, I'd favor an approach that takes it a step further by making a difficult thing easy for me.