See perlvar, where many special Perl variables are explained. Also see select.

Everyone should read perlintro when getting started with Perl, and refer back to it periodically, as it answers many basic questions and provides links to information for more in-depth ones. In this case, perlintro provides a link:

There are a number of "magic" scalars with names that look like punctuation or line noise. These special variables are used for all kinds of purposes, and are documented in perlvar.

And everyone posting here at PerlMonks should read Writeup Formatting Tips.

Now that you know what $| is, I suggest you don't use it! For Perl 5.8+, instead of your global LOGFILE file handle and:

select(LOGFILE); $| = 1;
select(STDOUT);
[download]

use IO::Handle;    # not needed for Perl 5.14+
# ...
$logfile->autoflush();
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Note that for Perl 5.14+ you don't need use IO::Handle because:

Before Perl 5.14, lexical filehandles were objects of the IO::Handle class, but you had to load IO::Handle explicitly before you could call methods on them. As of Perl 5.14, lexical filehandles are instances of IO::File and Perl loads IO::File for you.

Extra References Added Later

From On Interfaces and APIs:

Lexical file handles are, for me, the most important feature introduced in Perl 5.6. Those evil old global file handles spawned a host of unfortunate idioms, such as:

select((select($fh), $|=1)[0]);
[download]

In terms of a clear and simple interface to perform a simple task (set autoflush on a file handle), it doesn't get much worse than that, a compelling illustration of why keeping state in global variables is just plain evil ...

See also:

• Perl Best Practices, chapter 10 (I/O) by TheDamian
• Perl tip: Buffering and IO::Handle by TheDamian (tip for Melbourne Perl Mongers)
• Suffering from Buffering by MJD

PM References

• Is it possible at all to flush buffer and commit file write, if UTF-8 encoding is being used?? by dissident (2023)
• Re: Is it possible at all to flush buffer and commit file write, if UTF-8 encoding is being used?? - response from kcott
• Re: Is it possible at all to flush buffer and commit file write, if UTF-8 encoding is being used?? - response by ikegami

• Detect whether a writeable filehandle has closed? by jdporter (2021)
• Re: Detect whether a writeable filehandle has closed? (signal handlers) by me (with a Perl Monks history aside)

• Re^3: Telnet Client/Server: What am I doing wrong? by me (2012)
• Printing Unicode on the Windows Console and the importance of of i/o layers by nikosv (2010)

Unicode/UTF-8

• Encode (perldoc) - character encodings in Perl
• perlunitut (perldoc) - Perl Unicode Tutorial
• perlunifaq (perldoc) - Perl Unicode FAQ

• IO::File (perldoc) - supply object methods for filehandles
• IO::Handle (perldoc) - supply object methods for I/O handles
• utf8 (perldoc) - Perl pragma to enable/disable UTF-8 (or UTF-EBCDIC) in source code

• Win32::LongPath by Robert Boisvert (CPAN) - provide functions to access long paths and Unicode in the Windows environment

• Unicode (wikipedia)
• UTF-8 (wikipedia)
• Code Pages (Microsoft doco: Windows App Development)

• Emoji can be hard to see on the command line by kcott (2021) - see also Re: Add "Image Miniatures" to your HTML (Fun, Emoji and Ascii Art References)
• Re: Is it possible at all to flush buffer and commit file write, if UTF-8 encoding is being used?? by kcott (2023)
• Re^2: uparse - Parse Unicode strings (uparse/uchar/unichars/uniprops) by me (2023)

• Handling of Unicode File Names by NERDVANA (2023)
• how are ARGV and filename strings represented? by almr (2024) - see especially responses from ikegami
• Strings with umlauts and such by PeterKaagman (2024)

• Proper Unicode handling in Perl by VK (2019)

G'day Sriram,

i am unable to understand these two lines:

select(LOGFILE); $| = 1;
select(STDOUT);
[download]

The documentation for select has an example which is almost identical to the code you posted. It also has an example of autoflush() that was mentioned earlier in this thread.

If you familiarise yourself with the documentation available from http://perldoc.perl.org/perl.html, you'll get answers a lot quicker than you will by posting a question here and then waiting for replies.

One thing, that perhaps is not immediately apparent from the perlvar documentation, is that $| only has two states:

$ perl -le 'print $|; $| = 999; print $|; $| = ""; print $|'
0
1
0
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perldoc -v '$|'
[download]

Whenever you print data on a filehandle, it will be buffered and the data will not be availale in the file, until the buffer get full.

In Perl, we can not turn the buffering off, but you can get the same benefits by making the filehandle hot.

Here in you code select(LOGFILE); make the LOGFILE filehandle to be selected and $|=1, which is the special variable in perl, If you set it to a true value, it makes the current filehandle hot. So Perl will flush the data in the buffer immediately once you print on that filehandle.select(LOGFILE); select will return the currect filehandle.