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

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

sub extract{
    my $index = shift;
...
    }
    return undef;
}