If purchance it is necessary for you to keep the unique lines of your file in the same order, then this will remove all but the first occurance of each line and leave the remaining ones in their original order.

Just redirect the output to a new file on the command line (and uncomment the open line).

#! 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:)
[download]

Gives

C:\test>uniq
this is a line
this is another line
yet another
and yet another still
more
and more
and even more
and this
and that
but not the other cos its a family website:)

C:\test>
[download]

The caveat of course is that with a large file, that hash could get mind of big, but maybe that's ok if this is what you need to do.