#!/usr/bin/perl
use strict;

my $FileName = 'file.txt'; # Modify file name as needed.

my(@List,%List,@NewList)= ();

sub Abandon
{
print join '
',@_;
exit;
} # sub Abandon

print "Content-type: text/plain\n\n";

Abandon("Unable to read file $FileName") unless open R,"<$FileName";
@List = ;
close R;

Abandon("Unable to create temporary file ${FileName}.tmp.txt") unless open W,">${FileName}.tmp.txt";
for(@List) { print W $_; }
close W;

for(@List)
{
next if $List{$_};
$List{$_}++;
push @NewList,$_;
}

Abandon('Something wrong.',"Backup file is ${FileName}.tmp.txt") unless open W,">$FileName";
for(@NewList) { print W $_; }
close W;

unlink "${FileName}.tmp.txt";

print 'D O N E';