My code is just a special case.

A very, very special case. In your defined (@$_), you are trying to derefence $_ unconditionally. Considering you are using strict, this only succeeds in three cases: if $_ is an array reference, if $_ is an object with overloaded array dereferencing, or if $_ in undefined (but not in the general case, it seems only defined @$var, with $var undefined is allowed. @$var isn't, nor is defined @{+undef}). It's a run time to try to dereference a defined non-reference, or a reference that isn't an array (and doesn't have array reference overloading).

Why not just:

@flat = map {$_ ? @$_ : ()} @AoA;
[download]

if you're assuming @AoA has arrayrefs and undefined values, and nothing else.