It sounds like you might be on a UNIX or Linux system. If you are your system probably has syslog. You can send messages to syslog with the (standard) module Sys::Syslog. Your other program can watch for the log message to be sent to its destination.

Depending on how portable you want to be and whether the HUPs are to be sent by humans or software you can also use semaphores. These are bits accessible to multiple programs that can be used to send one bit messages (e.g. "I processed the file"). See IPC::Semaphore. One item you need to manage is when to set the bit back to zero. The semaphore system has ways to deal with this (because semaphores are in groups), but the best approach to use depends on how your daemon and the sender of the HUP interact.

It is interesting to note that while many programs do the log file or syslog thing, the sending programs often run "open loop" -- that is, they don't actually check that the update happened, they just assume it did. It is good to check it, though...

HTH, --traveler