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.
[download]

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;
[download]