54 characters, avoiding the recursive solution. 
sub h {
#23456789_123456789_123456789_123456789_123456789_1234
$x=0;$s=shift;$x={$_,[$x,0]}for sort{&$s($a,$b)}@_;%$x
}

[download]
Those complaining about strict.pm are invited to substitute $a and $b for $x and $s respectively. (Just to demonstrate that becoming strict does not necessarily result in clearer code...)

UPDATE
Updated per dragonchild's point about the spec misunderstanding. (How did this get updated? Well let me just say that after conferring with the author of the post, a kind janitor did the favour of editing it...)

In reply to Re (tilly) 1: (Golf) Building a Better Binary Tree by Anonymous Monk
in thread (Golf) Building a Better Binary Tree by Masem

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