Thanks for your explaination. It helps me understannd better.
That is why in the script, it reads 1024 bytes each time in a loop for each filehandel. For non-blocking filehandel, it has to be done by this kind of chunk-reading.
And using "while (<FH>)", is more for blocking filehandle: I mean, if "Open" not failed, for sure, you can read all lines from the filehandle.

My third question is about " EAGAIN() and retry'. I didn't undersntand this part of the code:

    $hl->{$_}->{retry} = 0;
    $hl->{$_}->{retries} = 0;

    my $start = time;
    my $blocksize = 1024;
    while (scalar keys %hltodo) {
        machine: for (keys %hltodo) {
            my $out = $hl->{$_}->{chld_out};
            # begin to read
            my $bytes_read = -1;
            while ($bytes_read) {
                my $buf;
                my $bytes_read = sysread($out, $buf, $blocksize);
                if (defined($bytes_read)) {
                    if ($bytes_read == 0) {
                        # eof
                        close($out);
                        last;
                    } else {
                $hl->{$_}->{data}.= $buf;
                    }
                } else {
                    if ($! == EAGAIN()) {
                        # retry
                        $hl->{$_}->{retry}++;
                        $hl->{$_}->{retries}++;
                        if ($hl->{$_}->{retry}) {
                            $hl->{$_}->{retry} = 0;
                            next machine;
                        }
                        usleep 10;
                    } else {
                        last;
                    }
                }
            }
            delete $hl->{$_}->{"chld_out"};
            delete $hltodo{$_};
        }
        # kill remaining pids if timeout reached
        if ($opt{timeout} && time > $start + $opt{timeout}) {
            print STDERR "Timeout for: ", join (" ", keys %hltodo), " 
+killing ", join (" ", values %hltodo) ;
            kill 1, values %hltodo;
            %hltodo = ();
        }
    }
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When the non-blocking filehandel is blocked dur to what ever the reason,it send$! to EAGAIN, then $hl->{$_}->{retry}++ will be 1, so it goes to " $hl->{$_}->{retry} = 0" and "next machine", ti will never do usleep 10 microsecond? I must miss something for this part?

EAGAIN means that whilst there is something available on the socket, hence select has given you it, that at the exact moment you tried to read it, something in the system or tcp stack was busy, and rather than block, it returns EAGAIN and lets you do something else in the mean time before trying again.

I agree with you that the retry logic in your code snippet is borked. It will only attempt one retry and will never do the usleep. What you choose to do about that is up to you. Personally, I think I'd probably omit the retry logic completely and just do the microsleep and loop back to the select; but you should probably consult someone with more *nix experience than me if that is your platform.

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