#! perl -sw
use strict;
my %lines;
#open DATA, $ARGV[0] or die "Couldn't open $ARGV[0]: $!\n";
while (<DATA>) {
    print if not $lines{$_}++;
}

__DATA__
this is a line
this is another line
yet another
and yet another still
this is a line
more
and more
and even more
this is a line
and this
and that
but not the other cos its a family website:)