You have many components so it's important to get each one working separately before trying them all togther

Try the script with
my $xml = do { local $/; <DATA> };
as I have provided and only if that works change it to
my $xml = $cgi->param("POSTDATA");

If it fails at the point it means that the XML being sent is not valid and we can move on to sorting out the client code

If the script I provided does not work then you have a bigger problem !