How about this: Think positive and use a regex for the fields you want rather than what might or might not separate them, like this:

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

my @fields =
  $part_sql =~ m{
    (?: ^ | , )      # start of string or comma
    (                # begin capture
      (?:            # either:
        [^,()]       #     non-paren non-comma
      |              # or:
        \(           #     left paren
        [^()]*       #     any amount of non-paren
        \)           #     right paren
      )*             # as often as possible
    )                # end capture
  }gx;               # take all matches

print map { "$_\n" } @fields;
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Of course, this simplistic approach does not handle nested parentheses.