Other thoughts to consider when evaluating return codes can include how the process was created. For example, if one uses the IPC::Open3 module to run the external command and the processes (pid) is killed using 'kill( 9, $pid )' the '$? >> 8' evaluation may result in 0. This is probably not the expected value, seeing how the processes was terminated. However, if one inspects the '${^CHILD_ERROR_NATIVE}' the result would show an exit value of 9.

According to PerlDoc:

If the filehandle came from a piped open, close returns false if one of the other syscalls involved fails or if its program exits with non-zero status. If the only problem was that the program exited non-zero, $! will be set to 0. Closing a pipe also waits for the process executing on the pipe to exit--in case you wish to look at the output of the pipe afterwards--and implicitly puts the exit status value of that command into $? and ${^CHILD_ERROR_NATIVE}.

http://search.cpan.org/~jesse/perl-5.12.2/pod/perlfunc.pod

Admittedly this may be a result of how things are terminated, closed and cleaned up. But it is a thought to consider.