G'day Manisha,

Welcome to the monastery.

I created two files:

$ cat ABC
qwe
asd
zxc

$ cat CDE
rty
fgh
vbn
[download]

I ran your code like this (apart from the shebang and three use lines, it's a copy of what you posted):

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

open(file_one,"ABC");
while(my $line = <file_one>)
 { open(file_two,"CDE");
     while(my $line2=<file_two>)
      { print "line : $line  line2 : $line2 \n";
      }
   close(file_two);
}
[download]

I got a warning but then output from both files, like this:

Name "main::file_one" used only once: possible typo at ./pm_example.pl
+ line 8.
line : qwe
  line2 : rty
 
line : qwe
  line2 : fgh
 
line : qwe
  line2 : vbn
 
line : asd
  line2 : rty
 
line : asd
  line2 : fgh
 
line : asd
  line2 : vbn
 
line : zxc
  line2 : rty
 
line : zxc
  line2 : fgh
 
line : zxc
  line2 : vbn
[download]

So, check the filenames, file permissions and file contents. If you ask for diagnostic messages, you'll usually get useful information: see Pragmas for documentation on those that I used; while you're learning, diagnostics can also be useful but don't leave it in production code.

Here's a better way to write it:

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

open my $fh1, '<', 'ABC';
while (defined(my $line1 = <$fh1>)) {
    chomp $line1;
    open my $fh2, '<', 'CDE';
    while (defined(my $line2 = <$fh2>)) {
        chomp $line2;
        print "line1 : $line1  line2 : $line2\n";
    } 
}
[download]

Now the output has no warnings and all those extra newlines are gone:

line1 : qwe  line2 : rty
line1 : qwe  line2 : fgh
line1 : qwe  line2 : vbn
line1 : asd  line2 : rty
line1 : asd  line2 : fgh
line1 : asd  line2 : vbn
line1 : zxc  line2 : rty
line1 : zxc  line2 : fgh
line1 : zxc  line2 : vbn
[download]

See the following for information on what I did differently to your posted code: chomp; defined; and open. Also, the following have useful information regarding while loops: "perlsyn: Loop Control" and "perlop: I/O Operators". Finally, if you've only just started to learn Perl, I recommend you first read all of "perlintro -- a brief introduction and overview of Perl".

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