I filed a ticket and was told that this is documented in perlxs.

This is quite useful as it usually improves readability. While this works fine for an SV *, it's unfortunately not as easy to have AV * or HV * as a return value. You should be able to write:

 AV *
 array()
     CODE:
         RETVAL = newAV();
         /* do something with RETVAL */
     OUTPUT:
         RETVAL
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But due to an unfixable bug (fixing it would break lots of existing CPAN modules) in the typemap file, the reference count of the AV * is not properly decremented. Thus, the above XSUB would leak memory whenever it is being called. The same problem exists for HV *.