But it's strange that only x doesn't flush automatically

Not any stranger than using x and print together. I'm not terribly surprised that this particular combination was not "expected". In any case, the code in question is:

            # Turn off the one-time-dump stuff now.
            if ($onetimeDump) {
                $onetimeDump      = undef;
                $onetimedumpDepth = undef;
            }
            elsif ( $term_pid == $$ ) {
                eval {          # May run under miniperl, when not ava
+ilable...
                    STDOUT->flush();
                    STDERR->flush();
                };

                # XXX If this is the master pid, print a newline.
                print $OUT "\n";
            }
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