my %seen;
    for ( my $i = 0; $i < scalar @arr; $i++){
        splice @arr, $i--, 1
            if $seen{$arr[ $i]}++;
    }
[download]

can be simplified to

    my %seen;
    for ( my $i = @arr; $i--; ){
        splice @arr, $i, 1
            if $seen{$arr[$i]}++;
    }
[download]

But the following will be much faster:

    my %seen;
    @arr = grep !$seen{$_}++, @arr;
[download]