RPC::XML is a perl implementation of the RPC-XML protocol, which is similar to SOAP(XML based remote proceedure calls.) Both can be served up via different transport protocols (HTTP, SMTP FTP) and are fairly similar in nature.
Something like the following:

1) User "A" submits a bug via the publicly acessible website.
2) Public website packages data into a SOAP call and calls SOAP server on bugtracking system in DMZ.
3) Soap server takes that object and interfaces with the bugtracking system's API to submit a new bug or trap an error.
4) If there is an error the SOAP server throws and exception which the client catches and displays something meaningfull to the user OR the bug is processed and some confirmation pages is displayed.

Would be a fairly simple implementation (SOAP::Lite|RPC::XML)