The following works:

#!/usr/bin/perl

use strict;
use warnings;

use Test::More tests => 5;
use Parse::RecDescent ();

my $p = Parse::RecDescent->new(<<'__END_OF_GRAMMAR__');

{
   use strict;
   use warnings;
}

parse    : rec /\Z/ { $item[1] }
rec      : POS_INT rec_list[ $item[1] ] { [ $item[0] => $item[2] ] }
rec_list : { $arg[0] == 0 ? [] : undef }
         | ELEM rec_list[ $arg[0]-1 ] { [ $item[1], @{$item[2]} ] }
POS_INT  : /\d+/
ELEM     : /\S+/

__END_OF_GRAMMAR__


ok($p->parse('0'));
ok($p->parse('1 foo'));
ok($p->parse('2 foo bar'));
ok(!$p->parse('1'));
ok(!$p->parse('1 foo bar'));
[download]

Changes:

• rec_list recursively builds a list from one element and a list. The terminating condition of the recursion is the count of items, which is passed as an argument to the rule.

• I added a check for end of "file" (/\Z/). This catches "1 foo bar". It's always good to check if you have leftover text to parse (unless you want to allow leftover text).

• I prefer <<'__END_OF_GRAMMAR__' over q{...} because it handles backslashes more intuitively.

• Your use strict and use warnings are in a different scope than the grammar. For them to apply to the grammar, you need to include them in the grammar.

• I renamed int to pos_int to make it clear that signs are not acceptable. A check on the numbers magnitude inside of rec wouldn't hurt.

• I uppercased tokens. It's just a style I use.

This is the same grammar as above, but with (yet-to-be-customized) error reporting:

my $p = Parse::RecDescent->new(<<'__END_OF_GRAMMAR__');

{
   use strict;
   use warnings;
}

parse     : rec eof { $item[1] }
eof       : /\Z/
          | <error>
rec       : POS_INT rec_list[ $item[1] ] { [ $item[0] => $item[2] ] }
rec_list  : rec_list_[ $arg[0] ]
          | <error>
rec_list_ : { $arg[0] == 0 ? [] : undef }
          | ELEM rec_list_[ $arg[0]-1 ] { [ $item[1], @{$item[2]} ] }
POS_INT   : /\d+/
ELEM      : /\S+/

__END_OF_GRAMMAR__
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.