Further info: I modified the "it's alive" to print out $! also and it's getting "No such process", but the process does exist and hasn't switched to any other user:

$ zt
cmd running: 18859
killing cmd: 18859
It's alive!!!!! (No such process)
It's alive!!!!! (No such process)
It's alive!!!!! (No such process)

$ ps aux
...
alan     18858  1.0  0.4   5660  1932 pts/2    S+   20:30   0:00 /usr/
+bin/perl ./zt
alan     18859  0.0  0.1   3184   492 pts/2    S+   20:30   0:00 sleep
+ 10
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