The server doing the POST expects XML back but I keep getting errors about "protocol violation".

You may want to trace a good request and a failing request with Wireshark and see where the differences lie. I don't know if this is related, but when I run the XML from your post through xmllint it complains about the entity in the payloadID, if I change that to a regular @ it doesn't complain. Anyway, here's an example with Mojolicious::Lite:

#!/usr/bin/env perl
use Mojolicious::Lite -signatures;
use Mojo::DOM; # in case you need to parse the incoming XML

sub TimeStamp { scalar gmtime } # dummy

post '/toMFF' => sub ($c) {
    my $body = $c->req->body;
    my $dom = Mojo::DOM->new->xml(1)->parse($body); # example
    say "[[[$dom]]]"; # just for debugging
    $c->render('test', format => 'xml',
        timestamp => TimeStamp=~s/\s/T/r);
};
 
app->start;

__DATA__
@@ test.xml.ep
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE cXML SYSTEM "http://xml.cXML.org/schemas/cXML/1.1.009/cXML.d
+td">
<cXML payloadID="foo@ceprinter.com" xml:lang="en-US" timestamp="<%= $t
+imestamp %>">
    <Response>
    <Status code="200" text="OK" />
    </Response>
</cXML>
[download]

Run this via morbo test.pl, I sent a test request via curl -X POST http://127.0.0.1:3000/toMFF -H "Content-Type: application/xml" -d "<foo><bar/></foo>" (I of course don't know if this is representative of your real-world requests), and I get the following response:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE cXML SYSTEM "http://xml.cXML.org/schemas/cXML/1.1.009/cXML.d
+td">
<cXML payloadID="foo@ceprinter.com" xml:lang="en-US" timestamp="WedTJu
+n  8 16:20:00 2022">
    <Response>
    <Status code="200" text="OK" />
    </Response>
</cXML>
[download]

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