Found in /usr/local/lib/perl5/5.00502/pod/perlfaq4.pod
  How can I extract just the unique elements of an array?

            There are several possible ways, depending on whether
            the array is ordered and whether you wish to preserve
            the ordering.

    a) If @in is sorted, and you want @out to be sorted:
    (this assumes all true values in the array)
                    $prev = 'nonesuch';
                    @out = grep($_ ne $prev && ($prev = $_), @in);

                This is nice in that it doesn't use much extra
                memory, simulating uniq(1)'s behavior of removing
                only adjacent duplicates. It's less nice in that it
                won't work with false values like undef, 0, or "";
                "0 but true" is ok, though.

    b) If you don't know whether @in is sorted:
                    undef %saw;
                    @out = grep(!$saw{$_}++, @in);