#!/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__