#!usr/bin/perl -w
use strict;

my %seen;
while (<DATA>)
{
   print unless $seen{$_}++;
}

#Prints:
#1 2 this is a line with 1 and 2
#3   a line with just 3
#5,6 five before 6
#5,6 five before six (different)
#6,5
#blah
__DATA__
1 2 this is a line with 1 and 2
3   a line with just 3
1 2 this is a line with 1 and 2
3   a line with just 3
5,6 five before 6
5,6 five before six (different)
6,5
blah
blah