#!/usr/bin/perl -w
    use strict;
    $_ = '$a ($a (hi) $a) $a (()$a)';
    my  $re;
    $re = qr/(?:[^\(\)]*|\((??{$re})\))+/;
    s/(\($re\))|\$a/$1?$1:'$b'/eg;
    print;


##</code><code>##

    my  $re;
    $re = qr/\((?:(?>[^()]*)|(??{$re}))*\)/;
    s/($re)|\$a/$1?$1:'$b'/eg;