Simplifying the grammar, we get:

use Parse::RecDescent;  
my $teststr="blah1,blah2(blah3,blah4(blah5,blah6(blah7))),blah8";  
my $grammar = q {
list: <leftop: item ',' item> 
item: word '(' list ')' <commit>
  | word 
word: /\w+/  
};  
my $parser = new Parse::RecDescent ($grammar) or die "Bad grammar!\n";
+  
defined $parser->list($teststr) or print "Bad text!\n";
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-- Randal L. Schwartz, Perl hacker