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

use Marpa::R2;

my $input = << '__INPUT__';
# This is a comment
feed_realtime_processor_pool = ( 11, 12 ) ;
dropout_detection_time_start = "17:00";
# Sometimes the config can have sub-structures
named_clients = (
{
  name = "thread1";
  user_threads = (
     { name = "realtime1"; cpu = 11; } # more comments
     { name = "realtime2"; cpu = 12; } # more comments
    );
}
);
__INPUT__

my $dsl = << '__DSL__';

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

Config    ::= Elements
Elements  ::= Element+                              action => grep_def
+ined
Element   ::= (Comment)                             action => empty
            | Name (s eq s) Value                   action => [values]
            | Name (s eq s) Value (semicolon s)     action => [values]
Comment   ::= (hash nonnl nl)                       action => empty
Name      ::= alpha
Value     ::= List
            | String
            | Num
            | Struct
List      ::= (lpar) Nums (rpar s semicolon s)
Nums      ::= Num+  separator => comma              action => listify
Num       ::= (s) digits (s)
            | (s) digits
            | digits (s)
            | digits
String    ::= (qq) nqq (qq semicolon s)             action => quote
Struct    ::= (lpar s) InStructs (rpar semicolon s)
InStructs ::= InStruct+                             action => grep_def
+ined
InStruct  ::= (lcurl s) Elements (rcurl s)
            | (Comment s)                           action => empty
            | Element

s         ~ [\s]*
eq        ~ '='
hash      ~ '#'
nonnl     ~ [^\n]*
nl        ~ [\n]
alpha     ~ [a-z_]+
lpar      ~ '('
rpar      ~ ')'
lcurl     ~ '{'
rcurl     ~ '}'
semicolon ~ ';'
comma     ~ ','
digits    ~ [\d]+
qq        ~ '"'
nqq       ~ [^"]+

__DSL__

sub listify      { shift; [ @_ ] }
sub quote        { qq("$_[1]") }
sub empty        {}
sub grep_defined { shift; [ grep defined, @_ ] }


my $id = 1;
sub show {
    my ($parent, $name, $elems) = @_;
    if (ref $elems->[0]) {
        show($parent, $name, $_) for @$elems;
    } elsif (ref $elems->[1]) {
        if (ref $elems->[1][0]) {
            say join ', ', $id, $parent, $elems->[0], 'null';
            show($id++, $elems->[0], $elems->[1]);
        } else {
            for my $e (@{ $elems->[1] }) {
                say join ', ', $id++, $parent, $elems->[0], $e;
            }
        }
    } else {
        say join ', ', $id++, $parent, @$elems;
    }
}


my $grammar = 'Marpa::R2::Scanless::G'->new({ source => \$dsl });
show('null', q(), ${ $grammar->parse(\$input, 'main') });
[download]

Output:

1, null, feed_realtime_processor_pool, 11
2, null, feed_realtime_processor_pool, 12
3, null, dropout_detection_time_start, "17:00"
4, null, named_clients, null
5, 4, name, "thread1"
6, 4, user_threads, null
7, 6, name, "realtime1"
8, 6, cpu, 11
9, 6, name, "realtime2"
10, 6, cpu, 12
[download]

Note that the id of the last line is 10, not 9 as in your sample. Adding leading zeroes and aligning the columns left as an exercise for the OP.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]