Perl kill != *nix kill. According to kill:

A negative signal name is the same as a negative signal number, killing process groups instead of processes. For example, kill '-KILL', $pgrp and kill -9, $pgrp will send SIGKILL to the entire process group specified. That means you usually want to use positive not negative signals.

With kill 'KILL', $cam_pid I got:

$ ./pm1054213.pl
cmd running: 7808
killing cmd: 7808
cmd closed off
dead and buried!
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