$ cat ABC
qwe
asd
zxc

$ cat CDE
rty
fgh
vbn

##</code><code>##

#!/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);
}

##</code><code>##

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
 

##</code><code>##

#!/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";
    } 
}

##</code><code>##

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