Perl Idioms Explained - keys %{{map{$_=>1}@list}} does caveat that this may not be appropriate for large lists. If that's a concern with the input being processed, the same thing can be done in line-by-line fashion.

perl -e '$seen{$_} = 1 while <>; print sort keys %seen' input.txt
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The article has some good commentary. For example, this approch above is slowest. Faster is this with $seen{$_} = undef, the grep approach is faster still, and the slice approach is apparently even faster.