On the one-liner, you are kicking out two child processes from the parent. Here's the code with indentation and decorations,

if (fork) {        #in parent
    if (fork) {    #in parent still
        waitpid -1, undef;
        print "-- ", $?>>8, "\n";
    }
    else {         # in second child
        exit 4;
    }
}
# else ???         # parent and first child drop off the end
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Unix does not guarantee the order of execution of parent and child. If the parent runs first, the second child may exit 4 before the first child does a normal exit by falling off the end of the program.

There is a good chance of your one-liner leaving zombie the child that waitpid didn't see. I/O generally takes more than one timeslice, so the remaining child is apt to end its brief life during print and shamble about waiting for somebody to read its exit status.