use strict;
use warnings;

use Data::Dumper qw( Dumper );
use XML::Simple  qw( );

local $XML::Simple::PREFERRED_PARSER = 'XML::Parser';

my $xs = XML::Simple->new(
   ForceArray   => [qw( elem subelem )],
   ForceContent => 1,
   #ContentKey  => '_text',
   KeyAttr      => {}, 
   KeepRoot     => 1,
);

my $tree = $xs->XMLin(<<'__EOI__');
<arr>
  <elem rp="rstring1">
    <mod>mstring1</mod>
    <subarr>
      <subelem>...</subelem>
      <subelem>...</subelem>
    </subarr>
  </elem>
  <elem rp="rstring2">
    <mod>mstring2</mod>
    <subarr>
      <subelem>...</subelem>
      <subelem>...</subelem>
    </subarr>
  </elem>
</arr>
__EOI__

{
   local $Data::Dumper::Indent = 1;
   print(Dumper($tree));
}

print $xs->XMLout($tree);