The problem stays even without prototypes.

Really? With a better prototype:

$ perl -wE 'sub list { 1, 2}; sub enum(\@@) { @_[1,2] = (1, 2)}; enum 
+my @a, state ($x, $y); say $x, $y'
12
[download]

Or with no prototype:

$ perl -wE 'sub list { 1, 2}; sub enum { @_[0,1] = (1, 2)}; enum my @a
+, state ($x, $y); say $x, $y'
12
[download]

To me this looks like the scalar context from the prototype was the problem - our maybe you meant a different problem? If so, care to elaborate?