My attempt at the negative PID didn't work. The complaint was "Bad signal number".

Your negative pid was taken to be the signal number. Explicitly specify the signal and you should be ok

$ perl -esleep &
[1] 3298

$ kill -3298
bash: kill: 3298: invalid signal specification

$ kill -KILL -3298

$
[1]+  Killed                  perl -esleep
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