You can attack this from both ends of the string using split with a third argument limiting the number of resultant fields and reverse.

#!/usr/local/bin/perl -l
#
use strict;
use warnings;

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

my @fields = split m{\s*,\s*}, $sql, 4;

splice @fields, 3, 1,
   reverse
   map { $_ = reverse }
   split m{\s*,\s*}, reverse($fields[3]), 2;

print for @fields;
[download]

Here's the output.

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

The use of spaces with commas was inconsistent so I decided to discard the spaces during the split.

Cheers,

JohnGG