$_ = "blah,blah(blah,blah(blah,blah(blah))),blah";
#$_="blah1,blah2(blah3,blah4(blah5,blah6(blah7))),blah8";
($re=$_)=~s/((\()|(\))|.)/$2\Q$1\E$3/gs;
@$ = (eval{/$re/});
die $@ if $@=~/unmatched/;
$re = join'|',map{quotemeta}@$;
print join"\n",/((?:$re|[^,])+)/g;