$_ = "a,b,op2(c,d),(e),(f),(f,g),op3(e,op4(f,g))\n";
print;
($re=$_)=~s/((\()|(\))|.)/$2\Q$1\E$3/gs;
@$ = (eval{/$re/});
die $@ if $@=~/unmatched/;
$re = join'|',map{quotemeta}@$;
print join"\n",/((?:$re|[^,])+)/g;