Ok it's definitely a problem with the proxy...
Set the backend running on its own with httpd -X, run a large (3MB) upload
and everything is fine - so no
mod_perl/embperl problem... (Thanks Gerald!)
Set both the backend and frontend running, both with httpd -X, run the same
upload:
Frontend appears to accept the upload, but returns a document with no data.
Backend bombs out with:
[error] CGI.pm: Server closed socket during multipart read (client aborted?)
After this, any access to the back-end (directly) returns no data at all
and is logged
as '200 - ' in the log.
This can be repeated consistently, so I guess this is a mod_proxy problem...
I have worked around by getting the front-end to run embpcgi.pl as a CGI
script for uploads
which (so far as I have tested) works fine...