For clarity, what you are calling an ordered associative array is actually an ordered relation. The definition of "associative array" is functional (ie: one value per key), not relational (ie: potentially N values per key). So we have: 
<ordered_relation_foo>
<b>2</b>
<b>4</b>
<a>1</a>
<b>6</b>
<a>3</a>
<a>5</a>
</ordered_relation_foo>

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becomes: 
$ordered_relation_foo={
    _meta_order=>[b,b,a,b,a,a],
    b=>[2,4,6],
    a=>[1,3,5]};

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So now a query of this ordered relation becomes: 
$ordered_relation_foo{a}

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How does that compare to the query of your structure?

In reply to Re^16: the annoying keys "key" and "value" by jabowery
in thread the annoying keys "key" and "value" by jabowery

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