my $expr = '((A,B),C,(D,E))';
print extract('C',$expr),"\n";
print extract('B',$expr),"\n";
print extract('D',$expr),"\n";

sub extract {
    my $char = shift;
    my ($str,$dup) = (shift) x 2;
    1 while $dup =~ s/\([^()$char]*\)/'.' x length $&/e;
    $dup =~ m/(\([^()$char]*$char[^()]*\))/;
    return substr($str,$-[1],length $1);
}
__END__
# output:
((A,B),C,(D,E))
(A,B)
(D,E)