Here's a (somewhat ugly) attempt using Text::Balanced. The idea is to extract the balanced parentheses fragments, in order to apply the split to the remaining parts of the string only.

use strict;
use warnings;
use Text::Balanced qw(extract_bracketed);

sub mysplit {
    my $text = shift;
    my @fields;
    do {
        my ($paren, $post, $pre) = extract_bracketed($text, '()', '[^(
+]*');
        
        my $s = '';
        if ($pre) {
            $s = $pre;
        } elsif (!$paren) {
            $s = $post;
            $post = '';
        }
        $s =~ s/^,//;            # get rid of superfluous leading comm
+a
        my @f = split /,/, $s;
        $f[-1] .= $paren if @f;  # append balanced parens part to last
+ elem
        
        push @fields, @f;
    
        $text = $post;
    } while ($text);
    return @fields;
}

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

print "$_\n" for mysplit($sql);
[download]

Output:

f1
f2
 SUM(f3)
CONCAT(f4,f5, f6)
 f7
[download]

(Not well tested — also, I have that feeling there must be something more elegant than this mess of conditionals... but it's escaping me right now :)