use strictures;
use WWW::Mechanize;
use URI;
use MIME::Base64;

my $endpoint = URI->new("http://bamboohost/rest/api/latest/result/projectname-planname-latest/label");
$endpoint->query_form( os_authType => "basic" ); # In case anonymonk is right.

my $xml = <<'';
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<label name="testlabel"/>

my $mech = WWW::Mechanize->new( autocheck => 0 );

$mech->default_header( Authorization => 'Basic ' . encode_base64( USER . ':' . PASSWORD ) );

my $response = $mech->post( $endpoint,
                            Content_Type => "application/xml",
                            Content => $xml );

print $response->as_string;