If you look in perlvar, you will see that $? is the status returned by the last closed pipe.

well, no, you have stoped reading too soon: 

$?  The status returned by the last pipe close, backtick
    (``) command, successful call to wait() or waitpid(),
    or from the system() operator.

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in my code, it is set to the exit code of the child by waitpid.

It is negated because by convention, exit code 0 means success and any other value failure. Also because this way I don't have to split $? into the signal part and the exit code: 

$signal = $? & 255;
$exit_code = $? >> 8;

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In reply to Re^4: Parallel::ForkManager problem by salva
in thread Parallel::ForkManager problem by Murcia

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