#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %hash = (
    key1 => 'value',
    key2 => undef,
    key3 => {},
    );

foreach my $key (qw(key1 key2 key3)) {
    print "\$hash{$key} exists: " . (exists $hash{$key} ? "yes" : "no") . "\n";
    print "\$hash{$key} is defined: " . (defined $hash{$key} ? "yes" : "no") . "\n";
    print "\$hash{$key} is defined and not exist: " . ((!defined $hash{$key} and !exists $hash{$key} ) ? "yes" : "no") . "\n";
}

__END__
$hash{key1} exists: yes
$hash{key1} is defined: yes
$hash{key1} is defined and not exist: no
$hash{key2} exists: yes
$hash{key2} is defined: no
$hash{key2} is defined and not exist: no
$hash{key3} exists: yes
$hash{key3} is defined: yes
$hash{key3} is defined and not exist: no