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my @list = qw/a b c d d a e b a b d e f/;
my @uniq = keys %{{ map {$_ => 1} @list }};
...

a b c d d a e b a b d e f
e f a b c d

map { $_ => 1 } @list

{ map { $_ => 1 } @list }

keys %{ { map { $_ => 1 } @list } }