Are you sure this isn't the problem:

if ($config{debug}) { print "pos[depth]: " . ${$pos[$depth]} . "\n" }
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This tries to treat $pos[$depth] as a scalar reference and dereference it. But that array entry is just a plain scalar, so you don't need to dereference it at all.

I'm guessing though, since I didn't take the time to run your code.