If you're doing that, you may as well do it in one pass: 
while (<>) {
  print unless $seen{$_}++;
}

[download]
You could also shrink the memory usage by computing your own hash value and using that as the %seen key -- but I don't think I'm going to get into any more details unless the original poster swears that this has nothing to do with harvesting addresses for spammers.

In reply to Re: Re: Removing duplicates in large files (a hash, or divide-and-conquer) by sfink
in thread Removing duplicates in large files by TIURIC

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