unable to write the output into a file in perl - perl

Question

I want to write the result of my code into a file , but it's not written into my file. My code is working and it's about removing duplicate line, but when i wanted to write the output into a file, the file is empty. This is my code:
use strict;
use warnings;
open(DATA,"/root/Desktop/SIEMENS/printtokens/outputs/common_distinct/MR1/thiscommon.txt");
open FILE2, ">/root/Desktop/SIEMENS/printtokens/outputs/common_distinct/MR1/common_element.txt" or die $!;
my %lines;
#open DATA, $ARGV[0] or die "Couldn't open $ARGV[0]: $!\n";
while (<DATA>) {
print if not $lines{$_}++;
print FILE2 if not $lines{$_}++;
}
close DATA;
close FILE2;

Answer 1

This works as expected
use strict;
use warnings;
use autodie;
open(my $in, '<', './in.txt');
open(my $out, '>', './out.txt');
my %lines;
while (<$in>) {
print $out $_ unless $lines{$_}++;
}
close $in;
close $out;
Note the usage of the 3-argument open, lexical filehandles and the autodie pragma.
Me as a fan of the usage of CPAN modules, would write the same as:
use 5.014;
use warnings;
use Path::Tiny;
use List::Util qw(uniq);
my $out_file = './out.txt';
my $in_file = './in.txt';
path($out_file)->spew( uniq path($in_file)->lines );

Answer 2

Your main problem probably is that $lines{$_}++ changes the value of $lines{$_} so the second will never evaluate to false. You could probably fix your problem just by removing ++ on the first occurence. But for readability I would recommend wrapping both output lines with one if statement, avoid using $_ and remembering to close the files, so something like:
use strict;
use warnings;
my %lines;
open(my $in, '<', './in.txt');
open(my $out, '>', './out.txt');
while (my $line = <$in>) {
unless ($lines{$line}++) {
print STDOUT $line;
print {$out} $line
}
}
close($in);
close($out);
As you can see, I also prefer lexical filehandles and 3-argument open.

Answer 3

Not quite sure what you want in common_element.txt, but I think the logic in the read loop is wrong. No need to increase the $lines twice, and only output if there is a match from existing data?
Try:
use strict;
use warnings;
open(DATA,"thiscommon.txt");
open (FILE2, '>common_element.txt') or die $!;
my %lines;
while (<DATA>) {
print FILE2 if $lines{$_};
print if not $lines{$_}++;
}
close DATA;
close FILE2;
Only with your paths, obviously.