Set the KeyAttr option for <nonterminal> to the empty list.

See XML::Simple for further details.

Update: quick explanation for my rather terse answer... when elements are repeated, XML::Simple puts them into either a hash or an array. It chooses a hash if it can find an attribute to use as the hash key. By default the attributes id, key and name are used. Perl hashes don't preserve order; Perl arrays do. By setting the key attribute list to the empty list, you force XML::Simple to use arrays.

That having all been said, in my experience for any even vaguely complex tasks, XML::Simple's "simplicity" actually ends up making things much harder. Witness the XML::LibXML solution...

use 5.010;
use XML::LibXML 1.94;
use PerlX::MethodCallWithBlock;

XML::LibXML::
    -> load_xml(location => 'myfile.xml')
    -> findnodes("//nonterminal | //token")
    -> map { $_->{name} || $_->textContent }
    -> foreach { say }
[download]

Hello, Z_S, and welcome to the Monastery!

Following the hints from daxim (since deleted) and tobyink (although I can’t find a solution that sets just the <nonterminal> KeyAttr option to the empty list), I came up with the following:

#! perl
use strict;
use warnings;
use XML::Simple;

my  $xml    = do { local $/; <DATA> };
my  $xmlObj = XML::Simple->new();
my  $data   = $xmlObj->XMLin($xml, KeyAttr => []);
my (@a_out, @ac_out);

foreach my $rule ( @{ $data->{rules}{rule} } )
{
    if    ($rule->{name} eq 'A')
    {
        push @a_out,  $_->{name} for @{ $rule->{nonterminal} };
    }
    elsif ($rule->{name} eq 'AC')
    {
        push @ac_out, $_->{name} for @{ $rule->{nonterminal} };
        push @ac_out, $_         for @{ $rule->{token}       };
    }
}

print join(' ', @a_out ), "\n";
print join(' ', @ac_out), "\n";

__DATA__
<detail>
  <start>
        <nonterminal name="A" />
  </start>
  <rules>
     <rule name="A" type="text">
        <nonterminal name="AA" />
        <nonterminal name="AC" />
        <nonterminal name="AD" />
        <nonterminal name="AB" /> 
        <nonterminal name="AE" />
     </rule>
     <rule name="AA" type="numeric">
        <token>1</token>
        <token>10</token>
     </rule>
     <rule name="AC" type="alpanumeric">
        <token>3</token>
        <nonterminal name="BE" />
        <nonterminal name="CE" />
        <token>30</token>
     </rule>
     <rule name="CE" type="special">
        <token>var1</token>
        <token>var2</token>
     </rule>
     <rule name="BE" type="symbol">
        <token>%</token>
        <token>#</token>
     </rule>
  </rules>
</detail>
[download]

Output:

AA AC AD AB AE
BE CE 3 30
[download]

I can’t see any way of retaining the interleaved ordering of rule AC’s <nonterminal> and <token> tags, though.

HTH,