I ran into the same problem and I don't see another way around the issue with CGI eating the input. The one difference is I found Pod::WSDL to generate my WSDL, so I have:

    my $pod = Pod::WSDL->new(
        source            => 'My::Module',
        location          => 'http://my/soap.cgi'
        pretty            => 1,
        withDocumentation => 1,
    );

    print CGI::header('text/xml');
    print $pod->WSDL;
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in my wsdl block. Did you end up using the code in your post?