I don't think this is a file upload. The vendor is going to "transmit the XML in some way" to port 443 on averylongloginname's server. Without knowing exaxtly how the vendor does this, makes it difficult to know what the answer is. i.e.:

1. Is the vendor doing a POST or GET to the URL provided?
2. Is the vendor opening a connection to port 443 and sending a stream of data (XML)

if (1), then what is the field name which will hold the XML. What other fieldnames are there that might be necessary?
if (2), will there be any headers\footers that need to be stripped off?

I suggest:
if (1) - then use the CGI module to capture the XML from the query_string or posted data and shove it in a variable to be parsed by your preferred method.
if (2) - then I'd suggest something like IO::Socket or HTTP::Daemon to "act" as a web server. I.e. listen on port 443 and wait for connections. See Listening on an HTTP port for some samples. But obviously this is made more difficult because you need to be able to deal with the SSL encoded data on your side.

averylongloginname, can you give some more specific details about how the XML is being sent? You can post it here, if you start another node then it will get confusing for everyone! =)

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