Showing us a sample of the code you have tried, or at least telling us what modules you are using, would help a great deal in providing a good answer.

A light weight way to perform your trick if you are not fussy about the order of the elements in the result is:

use strict;
use XML::Simple;

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

# parse the input string
my $root = XMLin ($org, KeepRoot => 1, );

# Insert the element
$root->{FILER}{LOCATION}{name} = '1stFloor';

# Dump the result
print XMLout ($root, RootName => undef);
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which prints:

  <FILER url="http://somewhere">
    <LOCATION name="1stFloor" />
    <PROJECT name="test_project" />
    <USER email="some@email.com" />
  </FILER>
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If you need to retain the original element order then you need something like XML::TreeBuilder:

use strict;
use XML::TreeBuilder;

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

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

$root->parse ($org);

# Insert the element
my $ins = XML::Element->new ('LOCATION', name => '1stFloor');

$root->push_content ($ins);

# Dump the result
print $root->as_XML ();
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which prints:

<FILER url="http://somewhere"><PROJECT name="test_project"></PROJECT><
+USER email="some@email.com"></USER><LOCATION name="1stFloor"></LOCATI
+ON></FILER>
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or XML::Twig:

use strict;
use XML::Twig;

my $org = <<'XML';
<FILER url='http://somewhere'><PROJECT name='test_project'/><USER emai
+l='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 ();
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which prints:

<FILER url="http://somewhere"><PROJECT name="test_project"/><USER emai
+l="some@email.com"/><LOCATION name="1stFloor"/></FILER>
[download]