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

use Data::Dumper;

my %hash =  ('a' => 2,  'b' => 3, 'c' => undef);
print Dumper (\%hash);

if (exists ($hash{'c'}) )
  {print "key c exists\n"}
else
  {print "key c does not exist\n"};
print Dumper (\%hash);

if ( defined ($hash{'c'}) )
  {print "key c  defined\n"}
else 
  {print "key c not defined\n"};
  
if (exists ($hash{'d'}) )
  {print "key d exists\n"}
else
  {print "key d does not exist\n"};

#note that undef,"",'' and 0 all evaluate to "false"
#play with c value above and run this code
#you can call defined($xxx) to figure out the difference
#between a false value from "",'',0 and undef.

if (my $x = $hash{'c'}) 
   {print "c has value $x\n"}
else
   {print "c has value,\"\",0 or undef\n"};

if (my $x = $hash{'b'}) 
   {print "b has value $x\n"}
else
   {print "b has no value\n"};
__END__
$VAR1 = {
          'c' => undef,
          'a' => 2,
          'b' => 3
        };
key c exists
$VAR1 = {
          'c' => undef,
          'a' => 2,
          'b' => 3
        };
key c not defined
key d does not exist
c has no value
b has value 3