push (@{ $hash{foo}{bar} }, @{ addSomeNumbersFromSource("data") });

If addSomeNumbersFromSource() is not changed from the OPed code, then the
    return @array;
statement will be evaluated in scalar context (dereferencing a scalar array reference) and will return the number of elements in @array.

tybalt89 has it right for the given code:
   push (@ { $hash{"foo"}{"bar"} } , @{ (addSomeNumbersFromSource("data"))[0] } );

Update:

Just return the reference, and let the caller dereference and handle the list.

Hmmm... On reading 1nickt's reply and re-reading the post to which I was replying, I must admit that that post urged a significant change to the function interface, for which the suggested expression invoking the function was perfectly appropriate. Paying attention to what's written can sometimes be useful. :)