As I said, I started from a wrong end. I'm kind of busy working from home and staying there with a wife and three children, so I didn't have time to fix it immediately. Here's a much simpler and faster version, which parses melpa's archive-contents in less than 5 seconds on my machine:

#! /usr/bin/perl
use warnings;
use strict;

use Marpa::R2;

my $dsl = << '__DSL__';

:default ::= action => ::first
lexeme default = latm => 1

List     ::= ('(') Elements (')')
Elements ::= Element+               action => [values]
Element  ::= List
           | Vector
           | Atom
           | String
           | Pair

Vector   ::= ('[') Elements (']')
Atom     ::= identifier
String   ::= ('"') Quoteds ('"')
Quoteds  ::= Quoteds Quoted         action => concat
           | Quoted
Quoted   ::= backslash
          || qq
          || plain
Pair     ::= Element (dot) Element  action => pair


:discard   ~ whitespace
whitespace ~ [\s]+

dot        ~ '.'
backslash  ~ '\\'
qq         ~ '\"'
identifier ~ [-\w@:+]+
plain      ~ [^\\"]+

__DSL__

sub concat { $_[1] . $_[2] }
sub pair   { +{ $_[1] => $_[2] } }

my $grammar = 'Marpa::R2::Scanless::G'->new({source => \$dsl});

my $lisp = do { local $/; <> };
my $value_ref = $grammar->parse(\$lisp, {semantics_package => 'main'})
+;
use Data::Dumper; print Dumper $value_ref;
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