An interesting note was made in the Chatterbox about how the idiomatic way of deleting duplicates won't work if you have both undef and the empty string ("") as values. This is because stringification treats them both the same.

It also won't work for an array of references, for the same reason. (It won't work for an ordered array for a different reason.)

In the spirit of fixing both issues, I offer the following:

sub unique (@)
{
    my ($arr) = shift;

    my %x;
    for my $index (0 .. $#$arr)
    {
        my $val = $arr->[$index];
        !defined($val) && do {
            $x{__NOT_DEFINED__} ||= [
                $index,
                undef,
            ];
            next;
        };

        $x{$val} ||= [
            $index,
            $val,
        ];
    }

    map { $_->[1] } sort { $a->[0] <=> $b->[0] } values %x;
}
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