#!/usr/bin/perl
use strict;
use warnings;

my $x;
my $y;

if ($y) {  }
if (! $y) {  }
if ($y && ! $y) {  }
if (exists $x->{notdef}) {  }
if (defined $x->{notdef}) {  }
if ($x->{notdef}) {  }

print ref $x, "\n";
print "\$y is undef\n" if ! defined $y;

if (exists $x->{notdef}{notdeftoo}) {}

print "Now \$x->{notdef} exists\n" if exists $x->{notdef};

##</code><code>##

HASH
$y is undef
Now $x->{notdef} exists