Here's a command-line variant of other solutions offered:

% perl -ne 'push @a, $_; $h{$_}++; END { print for grep $h{$_}==1, @a 
+}' foo.txt
[download]

If you don't care to preserve the original order of lines then:

% perl -ne '$h{$_}++; END { print for grep $h{$_}==1, keys %h }' foo.t
+xt
[download]