use strict;
use warnings;

{
   package TestHash;

   sub keyinfo {
      my ($k) = @_;
      if (defined($k)) {
         if (ref($k)) {
            print("reference\n");
         } else {
            print("string\n");
         }
      } else {
         print("undefined\n");
      }
   }

   sub TIEHASH  { my ($c      ) = @_; return bless([], $c);      }
   sub FETCH    { my ($s,$k   ) = @_; keyinfo($k); return undef; }
   sub STORE    { my ($s,$k,$v) = @_; keyinfo($k); push @$s, $k; }
   sub FIRSTKEY { my ($s      ) = @_; return shift @$s;          }
   sub NEXTKEY  { my ($s      ) = @_; return shift @$s;          }
}


tie my %h, 'TestHash';
foreach my $k ('a', [], undef) { $h{$k} = 1; }
print("\n");
foreach my $k ('a', [], undef) { my $dummy = $h{$k}; }
print("\n");
foreach my $k (keys %h) { TestHash::keyinfo($k); }