One possibility, although I am unsure how unconventional, is to return ($returnvalue | $signalled).

bash, for instance, returns 128+n if the command is terminated by signal n. It works well enough in practice... otoh, $returnvalue | $signalled is probably less useful:

$ perl -mPOSIX -E 'say for POSIX::SIGHUP, POSIX::EXIT_FAILURE'
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Another possiblity is to use a more sophisticated form of IPC, some kind of message queue, perhaps (but that's maybe overkill in this case).