@test = (23,23,4,8,21,90,90,90,90,2,2,2,19,21,19);
map {
    $length = ($test[$_ - 1] == $last)? $length + 1: 1;
    $run++ unless  $test[$_ - 1] == $last;
    $last = $test[$_ - 1];
    $runlengths[$run] = [$test[$_ - 1], $length];
} (1 .. scalar @test);

##</code><code>##

@strings = map { 
$runlengths[$_][0] . "x" . $runlengths[$_][1] } ( 1 .. $#runlengths);

##</code><code>##

sub extract{
    my $index = shift;
    my ($last, $lastindex);
    foreach (@runlengths[1 .. $#runlengths]){
	($last, $lastindex) = ($$_[0], $lastindex + $$_[1]);
	return $last if $index <= $lastindex - 1;
    }
    return undef;
}