lhoward's grammar seems unnecessarily complicated. Let's simplify it a bit, as well as grabbing what is needed for the answer (the split items):

use Parse::RecDescent; 
my $teststr="a,b,op2(c,d),op3(e,op4(f,g))"; 
my $grammar = q {
startrule: list
list: <leftop: item ',' item>
item: word '(' list ')' <commit> { "$item[1](".join(",",@{$item[3]})."
+)" }
    | word
word: /\w+/ 
}; 
my $parser = new Parse::RecDescent ($grammar) or die "Bad grammar!\n";
+ 
defined (my $result = $parser->startrule($teststr)) or print "Bad text
+!\n";
print map "<<< $_ >>>\n", @$result;
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Yes, there it is. $result is an array ref of the split-apart items.