use CGI(qw(:standard));
use strict;
...

if (param) {  ### HIER!
#Übergabeparameter
my $user=(param('user')||"");
...

$filename .= ".wsdl";
my $output = new IO::File(">$filename");
my $writer = new XML::Writer(OUTPUT => $output);
...
$writer->end();
...
$output->close();

}
else { #### UND HIER DANN
#öffnet html Code und weist ihn einer Variable zu
open (WSDL, "$html_path/addWSDL.html");
my $content=join ('',<WSDL>);
close WSDL;
...
print header();
print $content;
}