When a thread reaches the end of it's code block, or is issued a return, it sits and waits with it's return values, for the main thread to join it back in. When you detach the thread, the main thread no longer keeps count of it, and dosn't wait around for any return values. When a detached thread ends, it just goes away, instead of sitting there waiting to be joined.

If you are using the old 5.8.8 version of threads, you will have to do a bit more work, because, as you say, many thread methods are missing in 5.8.8. You will have to rely on older hacks and work-arounds, for instance see Re: Backticks and SIGALRM

Yes, the difference is whether or not you care about the final status (return code, etc.) of the thread.   If you do, it sits as a sort of “zombie” until you join it.   (The zombie status exists so that you can rendezvous without encountering race-conditions etc. ...)   If you don’t care, you can just detach the thread and it will perish on its own.

Thanks a lot, it works for me. May I bother you with another question? I saw some code like:

$pid = open my $fh, "$cmd |" or die "$!, $^E";

Could you please tell me what does it mean by '$^E'

I　google it, but nothing useful.

It's a built-in error message. Read perldoc perlvars and read the section on Error Variables. Its says:

 $EXTENDED_OS_ERROR
       $^E     Error information specific to the current operating system. At the moment, this differs from $! under only VMS, OS/2, and
               Win32 (and for MacPerl). On all other platforms, $^E is always just the same as $!.

               Under VMS, $^E provides the VMS status value from the last system error. This is more specific information about the last
               system error than that provided by $!. This is particularly important when $!  is set to EVMSERR.

               Under OS/2, $^E is set to the error code of the last call to OS/2 API either via CRT, or directly from perl.

               Under Win32, $^E always returns the last error information reported by the Win32 call "GetLastError()" which describes the
               last error from within the Win32 API. Most Win32-specific code will report errors via $^E. ANSI C and Unix-like calls set
               "errno" and so most portable Perl code will report errors via $!.

               Caveats mentioned in the description of $! generally apply to $^E, also.

               This variable was added in Perl 5.003.

               Mnemonic: Extra error explanation.

---

I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku ................... flash japh

I　google it, but nothing useful.

[ddg://"$^E" site:perl.org] -> "$^E" site:perl.org

[google://"$^E" site:perl.org] -> "$^E" site:perl.org

[google://"$^E" site:cpan.org] -> "$^E" site:cpan.org

[perldoc://"$^E"] -> "$^E"

Yeah, google seems to have gotten crappier, meh

perldoc -v "$^E"
    $EXTENDED_OS_ERROR
    $^E     Error information specific to the current operating system
+. At
...
[download]

$ perldoc perltoc |grep "\^E"
            , $COMPILING, $^C , $DEBUGGING, $^D , ${^ENCODING} , %ENV 
+,
            ${^CHILD_ERROR_NATIVE} , $EXTENDED_OS_ERROR, $^E ,
        $^E is meaningful on Win32
            $^E, $^H, $^M
        %ENV, CRTL_ENV, CLISYM_[LOCAL], Any other string, $!, $^E, $?,
+ $|
[download]

[doc://perlvar#$^E] -> $^E

You cant go wrong with a classic :) Tutorials: How to RTFM