From perldiag (I think this is only in 5.6, so perlmonk doesn't have it) :

No such pseudo-hash field ""%s""
(F) You tried to access an array as a hash, but the
field name used is not defined.  The hash at index 0
should map all valid field names to array indices for
that to work.
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Without seeing how your pseudohash is created I imagine that the structure of the array behind $foo, or its hash members is not what you think, or simply doesn't contain the 'blah' key.

Additionally, perl is probably interpreting @$foo as a pseudohash is because your sort routine references them as such :

$a->{'blah'} cmp $b->{'blah'}
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maybe

$$a->{'blah'} cmp $$b->{'blah'}
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would work, but I'm really just guessing there, since your data structure isn't apparent.