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);