Okay, I get where your going (I think). It work's but carries a fairly high cost.

I was premature in accepting etcshadow's suggestion. When it came to implementing it, the shine fell away.

This details the problem(s) as I see it so far:

Using your technique,

    @actual       %indexOf    @keyFor
0 [ A_value ]    { A => 0 }   [  A  ]
1 [ B_value ]    { B => 1 }   [  B  ]
2 [ C_value ]    { C => 2 }   [  C  ]
3 [ D_value ]    { D => 3 }   [  D  ]
4 [ E_value ]    { E => 4 }   [  E  ]
[download]

Accessing C via key calls for C to become top item so calling getByKey( 'C' ); does:

sub getByKey {
    my $key = shift;
    my $idx = $indexOf{ $key };
    my $value = $actual[ $idx ];
    push @actual, splice @actual, $idx, 1;
    push @keyFor, splice @keyFor, $idx, 1;
    
    ## Completely re-build %indexOf
    $indexOf{ $keyFor[ $_ ] } = $_ for 0 .. $#keyFor;
    
    return $value
}
[download]

Afterwards, all linkages are maintained:

    @actual       %indexOf    @keyFor
0 [ C_value ]*   { A => 1 }*  [  C  ]*
1 [ A_value ]    { B => 2 }*  [  A  ]*
2 [ B_value ]    { C => 0 }*  [  B  ]*
3 [ D_value ]    { D => 3 }*  [  D  ]*
4 [ E_value ]    { E => 4 }*  [  E  ]*
[download]

but requires two splices and the for loop is expensive!

---

Using etcshadow's layout, if I use splice to reorder @keys, the linkages in the hashes are self maintaining; but:

  @keys        %valByKey         %keyByValRef 
0 [ A ]    { A => \A_value }   { \A_value => A }
1 [ B ]    { B => \B_value }   { \B_value => B }
2 [ C ]    { C => \C_value }   { \C_value => C }
3 [ D ]    { D => \D_value }   { \D_value => D }
4 [ E ]    { E => \E_value }   { \E_value => E }
[download]

If accessing C via key, calls for C to become top item getByKey( 'C' ); does?

sub getByKey {
    my $key = shift;
    my $value = $valByKey{ $key };
    
    ## In order to splice the C item to the top
    ## I need it's index, but I don't have a way
    ## to go from key 'C' to index?
    
    return $value;
}
[download]

If there was a version of splice that located the items to remove by its address rather than by index, I would be laughing.

---

Using a hybrid of the two:

  @keys        %valByKey           %idxByVal
0 [ A ]    { A => \A_value }   { \A_value => 0 }
1 [ B ]    { B => \B_value }   { \B_value => 1 }
2 [ C ]    { C => \C_value }   { \C_value => 2 }
3 [ D ]    { D => \D_value }   { \D_value => 3 }
4 [ E ]    { E => \E_value }   { \E_value => 4 }
[download]

Accessing C via key calls for C to become top item getByKey( 'C' ); does:

sub getByKey {
    my $key = shift;
    my $value = $valByKey{ $key };
    my $idx = $idxByVal{ $value };
    push @keys, splice @keys, $idx, 1;
    
    ## To maintain linkage, it becomes necessary 
    ## to completely re-build %idxByVal
    
    %idxByValue = map{ $valByKey{ $key[ $_ ] } => $_ } 0 .. $#keys;
    
    return $value;
}
[download]

Afterwards:

  @keys        %valByKey           %idxByVal
0 [ C ]    { A => \A_value }   { \A_value => 0 }
1 [ A ]    { B => \B_value }   { \B_value => 1 }
2 [ B ]    { C => \C_value }   { \C_value => 2 }
3 [ D ]    { D => \D_value }   { \D_value => 3 }
4 [ E ]    { E => \E_value }   { \E_value => 4 }
[download]

One splice and a somewhat more expensive (because of double indirection) loop!

I'm still looking at the other two suggestions.

---

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