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

my ( $tok, @toks, $parens );

while ( $str ) {
  my $char = substr $str, 0, 1, '';
  
  $char eq ' ' and next;
  $char eq '(' and $parens++;
  $char eq ')' and $parens--; 
  $char eq ',' && ! $parens
               and push( @toks, $tok ), $tok = '', next;
  
  $tok .= $char;
  push @toks, $tok if ! $str;    
}

print join ' -- ', @toks;