my %hash = ("a" => undef, "b" => 0, "c" => 2);

testing("a");
testing("b");
testing("c");
testing("d");

sub testing {
	my $key = shift;
	print "$key exists\n" if (exists $hash{$key});
	print "the value of $key is defined\n" if (defined $hash{$key});
	print "the value for $key is true" if ($hash{$key});
}

##</code><code>##

a exists
b exists
the value of b is defined
c exists
the value of c is defined
the value for c is true