If the fork fails, $pid is undef, but will still compare true to 0. Second, if it fails, the error is in $!, not $pid.

Good catch! You are correct. My bad. Instead of

die "fork returned $pid";
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I should have invoked

die $!;
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as I have in code that I have written in the past where I have done forking. Too much C-code lately the if-tree is a C-ism. As in

pid = fork();
switch (pid){
-1: perror("Fork failed");
    exit(-1);
0: /* Child */
   break;
default:
    /* parent */
   break;
}
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anyway, good catch.

All all this code is off the top of my head.