use strict;
use XML::Twig;

my $org = <<'XML';
<FILER url='http://somewhere'><PROJECT name='test_project'/><USER email='some@email.com'/></FILER>
XML

# parse the input string
my $root = XML::Twig->new ();

$root->parse ($org);

# Insert a new element
my $ins = XML::Twig::Elt->new ('LOCATION', {name => '1stFloor'});

$ins->paste ('last_child', $root->root ());

# Dump the result
$root->print ();