@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  ]

##</code><code>##

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
}

##</code><code>##

    @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  ]*

##</code><code>##

  @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 }

##</code><code>##

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;
}

##</code><code>##

  @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 }

##</code><code>##

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;
}

##</code><code>##

  @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 }