How can I extract just the unique elements of an array?

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

    a) If @in is sorted, and you want @out to be sorted:

        $prev = 'nonesuch';
        @out = grep($_ ne $prev && (($prev) = $_), @in);

       This is nice in that it doesn't use much extra memory, 
       simulating uniq's behavior of removing only adjacent
       duplicates.

    b) If you don't know whether @in is sorted:

        undef %saw;
        @out = grep(!$saw{$_}++, @in);

    c) Like (b), but @in contains only small integers:

        @out = grep(!$saw[$_]++, @in);

    d) A way to do (b) without any loops or greps:

        undef %saw;
        @saw{@in} = ();
        @out = sort keys %saw;  # remove sort if undesired

    e) Like (d), but @in contains only small positive integers:

        undef @ary;
        @ary[@in] = @in;
        @out = sort @ary;