The method you are looking for is xml_string, which is also aliased as innerXML.

#!/usr/bin/perl

use strict;
use warnings;

use XML::Twig;

print "<foo>";
foreach my $file ( ' to_concat_1.xml', 'to_concat_2.xml')
  { print XML::Twig->new( keep_spaces =>1, comments => 'process')->par
+sefile( $file)->root->xml_string; }
print "</foo>\n";
[download]

Note that the comments => 'process' is used only because you have comments just before the end of the foo element in your example, it is probably not needed in your real code.

Also a more generic way would be to keep the first document, and then to add the other ones at the end, removing their root:

#!/usr/bin/perl

use strict;
use warnings;

use XML::Twig;

my $result_twig;

foreach my $file ( ' to_concat_1.xml', 'to_concat_2.xml')
  { my $current_twig= XML::Twig->new( comments => 'process')->parsefil
+e( $file);
    if( !$result_twig)
      { $result_twig= $current_twig; }
    else
      { $current_twig->root->move( last_child => $result_twig->root)
                           ->erase;
      }
  }

$result_twig->print;
[download]

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