http://search.cpan.org/author/DCONWAY/Parse-RecDescent-1.94/lib/Parse/RecDescent.pod#Autostubbing has

  function_call: identifier '(' arg(s?) ')'

  identifier: /[a-z]\w*/i
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as an example grammer, but

#! /usr/local/bin/perl -ws
use Parse::RecDescent;
my $parse = Parse::RecDescent->new(<<'EndGrammar');

    function_call: identifier '(' arg(s) ')'

    identifier: /[a-z]\w*/i

    arg: 'arg'

EndGrammar
while (<DATA>) { print $parse->function_call($_); }
__DATA__
a(1)
[download]

fails with

Use of uninitialized value in print at parse.pl line 18, 
<DATA> line 1.

Can anyone suggest what I'm doing wrong?