$|=1;

{
   local $^W = 0;
   print "1: ";
   print undef;        # no warning
   print "\n";
   print "1b: $^W\n";  # 0
   print "\n";
}

{
   local $^W = 1;
   print "2: ";
   print undef;        # warning
   print "\n";
   print "2b: $^W\n";  # 1
   print "\n";
}

{
   no warnings;
   print "3: ";
   print undef;        # no warning
   print "\n";
   print "3b: $^W\n";  # 0
   print "\n";
}

{
   use warnings;
   print "4: ";
   print undef;        # warning
   print "\n";
   print "4b: $^W\n";  # 0
   print "\n";
}

{
   use warnings;
   local $^W = 0;
   print "5: ";
   print undef;        # warning
   print "\n";         #   "use warnings" overrides "$^W = 0"
   print "5b: $^W\n";  # 0
   print "\n";
}

{
   no warnings;
   local $^W = 1;
   print "6: ";
   print undef;        # no warning
   print "\n";         #   "no warnings" overrides "$^W = 1"
   print "6b: $^W\n";  # 1
   print "\n";
}