DB<143> use warnings; no warnings 'uninitialized';  $h{+undef}
 => undef

  DB<144> sub tst { my $x=$_[0] }

  DB<145> use warnings; no warnings 'uninitialized'; tst $h{+undef}
 => undef

  DB<146> use warnings;  tst $h{+undef}
Use of uninitialized value in hash element at (eval 108)[multi_perl5db.pl:644] line 2.