use strict;
use warnings;

##</code><code>##

open(file1,"file1.txt") || die ("cannot open file");

##</code><code>##

open my $file1, '<', "file1.txt" or
  die "Can't open `file1.txt': $!\n";

##</code><code>##

while (<file1>)
   {
      chop();

##</code><code>##

      $REC     = $_;
      @LINEREC = split(/\,/,$REC);
      $data1 = @LINEREC[0];

##</code><code>##

while (<$file1>)
   {
      chomp;
      my $data1 = (split /,/)[0];
      # ...

##</code><code>##

   open(file2,"file2.txt") || die ("cannot open file");

##</code><code>##

#!/usr/bin/perl -l

use strict;
use warnings;

my ($fh1, $fh2) = map {
    open my $fh, '<', $_ or
      die "Can't open `$_': $!\n"; 
    $fh } qw/file1.txt file2.txt/;

while (<$fh1>)
{
    chomp;
    my $data1 = (split /,/)[0];
    seek $fh2, 0, 0;
    while (<$fh2>) {
        chomp;
        print $data1 if 
          $data1 eq (split /,/)[1];
    }
}

__END__