Mmmm, I look at that code and I'm really not sure what it's doing without really having to think about it. Maybe it's my aversion to for (;;) style loops but something more like this seems much more understandable to me:

my (%values, $key);
foreach my $value ( split /[&;]/, $query_string ) {
    my ($name, $data) = split /=/, $value;
    if ( lc($name) eq 'o' ) {
        $key = $data;
        $values{$key} = undef 
            unless exists $values{$key};
    } elsif ( lc($name) eq 'd' and defined $key ) {
        # anything you need to do to $data happens here
        if ( ref $values{$key} eq 'ARRAY' ) {
            push @{$values{$key}}, $data;
        } elsif ( defined $values{$key} ) {
            $values{$key} = [ $values{$key}, $data ];
        } else {
            $values{$key} = $data;
        }
    }
}
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It does throw away anything that isn't either an o or d param and any d that occurs before the first o is also thrown away but as your code also does that it seemed the thing to do.

And if you always use array refs then the second half of the d/o if statement becomes:

    push @{$values{$key}}, $data;
[download]

which seems to me all the clearer.

Struan