It was finally not that difficult, in case you encounter something similar:
You only need to add the requesting IP as _location to the http header.
Thanks!

#!/usr/bin/perl

use warnings;
use strict;
use Data::Dumper;
use WWW::Mechanize;
use CGI  qw/:standard/;
use CGI::Carp qw(warningsToBrowser fatalsToBrowser);

my $q = new CGI;
my $addr = $ENV{'REMOTE_ADDR'};

my $m = WWW::Mechanize->new();
my $url = 'http://projects.villa-bosch.de/dbase/molsurfer/submit-elec.
+html';


$m->get($url);
$m->add_header(location => $addr);
$m->set_fields(
       FILE1 => $file1,
       FILE2 => $file2,
       mapsize =>'59x59',
       resolution =>'med',
       pdie =>'4',
       sdie =>'80',
       ioss =>'150',
       ionr =>'1.5',
       nmap =>'0.0'
      );  
my $response = $m->submit();
print $q->header;
print $response->content();
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