The status returned by the last pipe close, backtick (``) command, successful call to wait() or waitpid(), or from the system() operator. This is just the 16-bit status word returned by the wait() system call (or else is made up to look like it). Thus, the exit value of the subprocess is really ($? >> 8), and $? & 127 gives which signal, if any, the process died from, and $? & 128 reports whether there was a core dump. (Mnemonic: similar to sh and ksh.)
Additionally, if the h_errno variable is supported in C, its value is returned via $? if any gethost*() function fails.
If you have installed a signal handler for SIGCHLD, the value of $? will usually be wrong outside that handler.
Inside an END subroutine $? contains the value that is going to be given to exit(). You can modify $? in an END subroutine to change the exit status of your program. For example:
END {
$? = 1 if $? == 255; # die would make it 255
} Under VMS, the pragma use vmsish 'status' makes $? reflect the actual VMS exit status, instead of the default emulation of POSIX status.
Also see "Error Indicators".