emacsclient: controlling from an application?

[DrMemory]

I would like to have a cron task signal emacsclient to save a certain
buffer which I keep open throughout the workday, so that a perl script
can do some processing on the contents at the end of the day.
Signalling to save _all_ buffers would be okay too.

I know I can set the auto-save-visited-file-name variable so that
auto-saves write to the original file; but unless I could do this on a
per-buffer basis I surely wouldn't want this as global behaviour.