A Typical computer program is run from a single keyboard or other control point, but with access to thousands of files. So it's "cheap" for a programming language to open a connection and provide a buffer to that control centre (often known as "standard in" and "standard out", but impractically expensive to have connections open to all the available files. For this reason, most languages provide a function with a name such as open or fopen to provide a resource through which files can be read and written.

Perl is like this but (being Perl) goes a step further. The following file handles are open by default:
STDIN - Standard Input, for user control input, typically the keyboard
STDOUT - Standard Output,for regular use output, typically the screen
STDERR - Standard Error, for error output; typically defaults to the screen too
DATA - Input for data stored after __END__ at the end of the program

Here's an example program that uses all four of these special file handles:

while (<DATA>) {

        chop;

        $mname{$_} = ++$mn;

        }



# seaweed - - [15/Jul/1998:08:32:38 -0400] "GET / HTTP/1.0" 200 1476



print "Which Host? ";

chop ($host = <STDIN>);



while (<>) {



        ($day, $month, $year,$page) = 

        m!^$host\s.*\[(\d{2})/(...)/(....).*(?:POST|GET)\s+(\S+)!;



        if ($day) {

                if ($page !~ m!^/!) {

                        print STDERR "YIKES ... $page\n";

                }

                print STDOUT "$day - $mname{$month} - $year .... $page\n";



        }

}

__END__

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STDERR merits a short extra comment. You may wonder why there are two outputs both of which, by default, are routed to the screen. The "trick" is that it's useful to have two separate file handles for use then STDOUT is diverted away from the screen - for example, when STDOUT is diverted to a browser or a file. In such a circumstance, the user typically doesn't want error messages going to the same place, but wants it to go to the server's error log, or to the screen, respectively. You'll find more examples of Perl's file handling in our File Handling module.
(written 2005-03-23, updated 2006-06-05)

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STDIN, STDOUT, STDERR and DATA - Perl file handles
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