XML::Simple folds (by default) on the name= attribute. At lines -3, -8 from the end you have two <members> lines with the same name="AttributeDesc". This fooled XML::Simple into treating the two blocks as the same, allowing the second instance to overwrite the first. By adding KeyAttr=>[] as an option to XMLin, this default behaviour is suppressed and the expected result is obtained.

use feature ":5.14";
use warnings FATAL => qw(all);
use strict;
use Data::Dump qw(dump pp);
use XML::Simple;

my $x = XMLin(<<'END', KeyAttr=>[]);
  <classes name="Panoply::AttributeDesc">
    <all_members name="AttributeDesc" protection="public" scope="Panop
+ly::AttributeDesc" virtualness="non_virtual"/>
    <all_members name="AttributeDesc" protection="public" scope="Panop
+ly::AttributeDesc" virtualness="non_virtual"/>
    <all_members name="description" protection="public" scope="Panoply
+::AttributeDesc" virtualness="non_virtual"/>
    <all_members name="name" protection="public" scope="Panoply::Attri
+buteDesc" virtualness="non_virtual"/>
    <all_members name="value" protection="private" scope="Panoply::Att
+ributeDesc" virtualness="non_virtual"/>
    <public_members>
      <members kind="variable" name="name" protection="public" static=
+"no" type="std::string" virtualness="non_virtual"></members>
      <members kind="variable" name="value" protection="public" static
+="no" type="std::string" virtualness="non_virtual"></members>
      <members kind="variable" name="description" protection="public" 
+static="no" type="std::string" virtualness="non_virtual"></members>
    </public_members>
    <public_methods>
      <members const="no" kind="function" name="AttributeDesc" protect
+ion="public" static="no" virtualness="non_virtual" volatile="no">
        <parameters declaration_name="name" type="const std::string &a
+mp;"/>
        <parameters declaration_name="value" type="const std::string &
+amp;"/>
        <parameters declaration_name="desc" default_value="&quot;&quot
+;" type="const std::string &amp;"/>
      </members>
      <members const="no" kind="function" name="AttributeDesc" protect
+ion="public" static="no" virtualness="non_virtual" volatile="no"></me
+mbers>
    </public_methods>
  </classes>
END

#pp($x);

sub r($$$;$);
sub r($$$;$)
 {my ($r, $l, $e, $a) = @_;
  $a = [] unless $a;
  return unless $l and ref($l); 
  if (ref($l) =~ /HASH/)
   {for(sort keys %$l)
     {unless (/$e/)
       {push @$a,  $_;
        r($r, $l->{$_}, $e, $a);
        pop @$a;
       }
      else
       {&$r(@$a);
       }
     } 
   } 
  elsif (ref($l) =~ /ARRAY/)
   {for(1..@$l)
     {unless ($l->[$_-1] =~ /$e/)
       {push @$a,  $_;
        r($r, $l->[$_-1], $e, $a);
        pop @$a;
       }
      else
       {&$r(@$a);
       }
     } 
   } 
 }

r sub {say "@_"}, $x, "parameters";
[download]

Produces

public_methods members 1