#!/usr/bin/perl -w
use strict;

my %hash = (a => 1,
	    b => 0,
	    c => undef);

foreach ('a' .. 'd') {
  print "\$hash{$_) : ";

  if (exists $hash{$_}) {
    if (defined $hash{$_}) {
      if ($hash{$_}) {
	print "exists, defined & true\n";
      } else {
	print "exists, defined & false\n";
      }
    } else {
      print "exists, undefined & false\n" 
    }
  } else {
    print "non-existant, undefined & false\n";
  }
}