$part_sql = "f1,f2, SUM(f3),CONCAT(f4,f5, f6), f7"

##</code><code>##

my @fields = split(/(?<!\([^\)]+),/, $part_sql);

##</code><code>##

Variable length lookbehind not implemented in regex; marked by <-- HERE in m/(?<!\([^\)]+), <-- HERE / ....