local $_ = "this, (that, those), these ((not enough, (nope)), never), there";
(my $re=$_)=~s/((\()|(\))|.)/${[')','']}[!$3]\Q$1\E${['(','']}[!$2]/gs;
$re= join'|',map{quotemeta}eval{/$re/};
die $@ if $@ =~ /unmatched/;
while( /((?:$re|[^,])*)/g ){
    print "$1\n";
}