devarishi:

Here's a trivial example. Suppose you have a program that generates some output, but you'd like the output sorted. You can do it like so:

use strict;
use warnings;
open my $F1, '| sort >t' or die;
while (<DATA>) {
    print $F1 $_;
}
__DATA__
the
quick
red
fox
jumped
over
the
lazy
brown
dog
[download]

This trivial program just writes a few words, and then uses the sort program to sort the output using the sort program as a filter. When you run it, you get this:

$ perl foo.pl
$ cat t
brown
dog
fox
jumped
lazy
over
quick
red
the
the
[download]

...roboticus

When your only tool is a hammer, all problems look like your thumb.

roboticus: Thanks a lot! Based upon my understanding of your program above, I wrote this small one:

#!/usr/bin/perl -w
open ($FH,'| sort -n') or die($!);
while(<ARGV>){
        print $FH $_;
}
close $FH;
[download]

Thanks again for "__DATA__" as well. There is one thing I would like mention: I used to write FH instead of $FH in open() as the file-handler. But both of them work. I did not know it. Please have a look at this:

[demo@localhost Perl]$ cat bar.pl
#!/usr/bin/perl -w
open (FH,'| sort') or die($!);
$count=0;
while(<ARGV>){
    print FH $_;
    print $count++ . "\n";
}
close FH;

[demo@localhost Perl]$ bar.pl words 
0
1
2
3
4
A
ABC
a to z
Buy
Why
[download]

The line numbers are being printed out first and then the sorted list of the words. That means (this is what I understand now) that the data are being fed to the "sort" command which returns its result when all the input to it is fed.

devarishi:

Yes, it sounds like you've got a handle on it, now. Note that you can also have a filter on the input, too. So if you wanted to process the records in sorted order, you could open the incoming data stream with a filter to do so.

Regarding the $FH file handle versus the FH version: Modern perl practice discourages the old style (FH) style. The older style continues to work, obviously, but using a file handle in a scalar allows you a bit more flexibility in scoping and a bit more simplicity when passing the file handle to subroutines and such.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Otherwise tell us what you want or need to actually do, write to the filehandle, read from it?, both? Those |'s are shortcut notation for pipes.