Yes, you're absolutely correct - nice catch :)

Here's an updated one-liner that addresses that problem in the way you suggest:

perl -ne '$x{$_}++;}{for(sort keys %x){print if $x{$_}==1;}' < input.t
+xt
[download]

(I'm not a golfer by any strech of the imagination, so I imagine that could be shortened significantly)

Cheers,
Darren :)