my (@out,$ref) = undef;
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My guess as to the intent of this statement from the OPed code is that it was to be some kind of self-documentation of the creation of two lexical variables: an empty array and an undefined scalar. It does not do what I think you think it does.

In fact, it is equvalent to the
    my (@out, $ref) = (undef);
statement, which uses the list  (undef) to initialize the newly-created lexicals. However, due to list flattening, the  @out array 'consumes' all the elements of the initialization list, thus producing a non-empty array with a single element: undef. In view of the  push @out, ...; statement in the subsequent for-loop, it seems unlikely this behavior was intended. (But the  $ref scalar remains undefined after all this!)

>perl -wMstrict -MData::Dump -le
"my (@out,$ref) = undef;
 dd \@out;
"
[undef]
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