is missing the correct operator between POST and the URL string

Sure its not :)

use DDS;
use HTTP::Request::Common qw(POST);
Dump( POST 'http://example.com', [ qw' username joe password joe ' ] )
+;
__END__
$HTTP_Request1 = bless( {
                   _content => 'username=joe&password=joe',
                   _headers => bless( {
                                 "content-length" => 25,
                                 "content-type"   => 'application/x-ww
+w-form-urlencoded'
                               }, 'HTTP::Headers' ),
                   _method  => 'POST',
                   _uri     => \do { my $v = 'http://example.com' }
                 }, 'HTTP::Request' );
bless( $HTTP_Request1->{_uri}, 'URI::http' );
[download]

I agree with the rest of your words