Can't kill a non-numeric process ID at ./t.pl line 36.

Being as the script I linked to only has 35 lines, you must have changed something.

Given the reference to "kill", it would have to be one of these two lines producing the error:

    kill 0, $pid while sleep 1 and $timeout--;
    kill 3, $pid and return if $timeout;
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And given that $pid is set by this line:

        $pid = open my $fh, "$cmd |" or die "$!, $^E";
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it can only take a numeric value or undef. If it was undef, due to the command failing to run, then the die would have prevented the code reaching the kill lines.

So, what did you change and why?