#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use Marpa::R2;
use Data::Dumper;

my $input = q(sys ecm cloud-provider /Common/aws-ec2 {
    description "The aws-ec2 parameters"
    property-template {
        account { }
        availability-zone {
            valid-values { a b c d }
        }
        instance-type {
            valid-values { t2.micro t2.small t2.medium }
        }
        region {
            valid-values { us-east-1 us-west-1 }
        }
    }
});

my $dsl = << '__DSL__';

lexeme default = latm => 1

:default ::=                       action => first
Top      ::= atom Attrs Struct     action => top
Attrs    ::= atom Attrs            action => merge
           | atom                  action => newlist
Struct   ::= ('{') Elements ('}')
Elements ::= Element Elements      action => merges
           | Element
Element  ::= atom Value            action => struct
           | atom Struct           action => struct
Value    ::= Struct
           | ('"') string ('"')
           | ('{ }')               action => empty
          || ('{') List ('}')
List     ::= atom List             action => merge
           | atom                  action => newlist

:discard ~ [\s]
string ~ [^"]+
atom   ~ [^\s{}]+

__DSL__

sub top     { +{ $_[1] => { attrs => $_[2], contents => $_[3] } } }
sub first   { $_[1] }
sub empty   { [] }
sub newlist { [ $_[1] ] }
sub merge   { [ $_[1], @{ $_[2] } ] }
sub struct  { +{ $_[1] => $_[2] } }
sub merges  { +{ %{ $_[1] }, %{ $_[2] } } }


my $grammar = 'Marpa::R2::Scanless::G'->new({ source => \$dsl });
my $recce = 'Marpa::R2::Scanless::R'->new({ grammar           => $gram
+mar,
                                            semantics_package => 'main
+',
                                          });
$recce->read(\$input);
use Data::Dumper; print Dumper($recce->value) =~ s/  / /gr;
[download]

$VAR1 = \{
   'sys' => {
      'contents' => {
          'property-template' => {
                'account' => [],
                'region' => {
                    'valid-values' => [
                         'us-east-1',
                         'us-west-1'
                        ]
                   },
                'availability-zone' => {
                       'valid-values' => [
                            'a',
                            'b',
                            'c',
                            'd'
                           ]
                      },
                'instance-type' => {
                      'valid-values' => [
                           't2.micro',
                           't2.small',
                           't2.medium'
                          ]
                     }
                },
          'description' => 'The aws-ec2 parameters'
          },
      'attrs' => [
         'ecm',
         'cloud-provider',
         '/Common/aws-ec2'
         ]
      }
   };
[download]

Update: Fixed the missing + in the top rule, compacted the output, reverted the order of the merge rule.

Update2: Added the default action.