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)

In reply to (jeffa) 3Re: Removing Duplicate Lines from a File by jeffa
in thread Removing Duplicate Lines from a File by johnirl

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