If you run DamnDirtyApe's excellent one-liner through Deparse:

perl -MO=Deparse -ne 'print unless $n{$_}++' my_file.txt
[download]

you will see how:

LINE: while (defined($_ = <ARGV>)) {
    print $_ unless $n{$_}++;
}
[download]

You can remove the LINE: label and you should add use strict.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)