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

{
   use strict;
   use warnings;
}

parse    : rec /\Z/ { $item[1] }
rec      : POS_INT
           {
              $thisparser->_parserepeat(
                 $text,
                 \&ELEM, $item[1], $item[1],  # ELEM(#)
                 $_noactions, $expectation, undef
              )
           }
           { [ $item[0] => $item[2] ] }
POS_INT  : /\d+/
ELEM     : /\S+/

__END_OF_GRAMMAR__

##</code><code>##

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] < 1 ? [] : undef }

         | { $arg[0] < 10 ? 1 : undef }
           ELEM rec_list[ $arg[0]-1 ]
           { [ $item[2], @{$item[3]} ] }

         | { $arg[0] < 100 ? 1 : undef }
           ELEM(10) rec_list[ $arg[0]-10 ]
           { [ @{$item[2]}, @{$item[3]} ] }

         | { $arg[0] < 1000 ? 1 : undef }
           ELEM(100) rec_list[ $arg[0]-100 ]
           { [ @{$item[2]}, @{$item[3]} ] }

         | <error:Exceeded maximum list length of 999 elements>

POS_INT  : /\d+/
ELEM     : /\S+/

__END_OF_GRAMMAR__