use strict ;
    use warnings ; # ...if you have Perl 5.6

    # read from stdin, spit data to stdout
    # (just to keep it simple)
    use Digest::MD5 qw(md5_hex) ; # or one of md5*'s

    my %digests ;

    while (my $line = <STDIN>) {
      my $dig = md5_hex($line) ;
      if (exists $digests{$dig}) {
        print STDERR "WARNING: duplicated checksum $dig for line $line\nWARNING: skipping $line\n" ;
         $digest{$dig}++ ; # you can use this to count repetitions
      } else {
        $digest{$dig} = 0 ;
        print $line ;
      }
    }