This is all what trace shows me:

Accept: text/xml
Accept: multipart/*
Accept: application/soap
Content-Length: 578
Content-Type: text/xml; charset=utf-8
SOAPAction: "http://localhost/MYSOAP#mySub"

<?xml version="1.0" encoding="UTF-8"?><soap:Envelope xmlns:wsdlsoap="h
+ttp://schemas.xmlsoap.org/wsdl/soap/" soap:encodingStyle="http://sche
+mas.xmlsoap.org/soap/encoding/" xmlns:soap="http://schemas.xmlsoap.or
+g/soap/envelope/" xmlns:tns1="http://localhost/MYSOAP" xmlns:wsdl="ht
+tp://schemas.xmlsoap.org/wsdl/" xmlns:soapenc="http://schemas.xmlsoap
+.org/soap/encoding/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-inst
+ance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><soap:Body><tns1:m
+ySub><foo xsi:type="xsd:string">HELLO</foo></tns1:mySub></soap:Body><
+/soap:Envelope>
SOAP::Transport::HTTP::Client::send_receive: HTTP/1.1 200 OK
Connection: close
Date: Wed, 29 Sep 2010 11:43:07 GMT
Accept-Ranges: bytes
Content-Length: 226
Content-Type: text/plain; charset=utf-8
Last-Modified: Wed, 29 Sep 2010 09:17:08 GMT
Client-Date: Wed, 29 Sep 2010 11:43:07 GMT
Client-Peer: IP:80 
Client-Response-Num: 1

use lib qw(/path/to/my/module);
use SOAP::Transport::HTTP;

  SOAP::Transport::HTTP::CGI   
    -dispatch_to('MYSOAP')
#    ->dispatch_with({ 'urn:localhost:wsdl:MYSOAP' => 'MYSOAP' })
    -> handle;



syntax error at line 1, column 0, byte 0 at /usr/lib/perl5/XML/Parser.
+pm line 190
use lib qw(/path/to/my/module);
use SOAP::Transport::HTTP;

  SOAP::Transport::HTTP::CGI   
    -dispatch_to('MYSOAP')
#    ->dispatch_with({ 'urn:localhost:wsdl:MYSOAP' => 'MYSOAP' })
    -> handle;
[download]

As you see there is my server code shown up, which is stored in $arg and i thought that here should be the xml part which is at the beginning of the output

the WSDL file was created by Pod::WSDL so i think that should be valid but i'm going to check this out.
I checked the xml Output which trace shows me and it is valid xml

Thanks so far.