Interestingly, your warning is still in effect. The only way I can throw it is through this bug:

# OK, gets implied defined(), no warning, correct output
% perl -lwe 'print while glob("{dog,cat,0,pig,cow}")'

  dog
  cat
  0
  pig
  cow


# Buggy, no implied defined(), throws warning and misbehaves
% perl -lwe 'print while $_ = glob("{dog,cat,0,pig,cow}")'

  Value of glob construct can be "0"; test with defined() at -e line 1
+.
  dog
  cat
[download]

-Blake