# If record 'd' does not exist:
if (!$hash{'d'})

##</code><code>##

# If record 'd' does not exist, or
# if record 'd' exists but has no field 'value':
if (!$hash{'d'} || !exists($hash{'d'}{'value'}))

##</code><code>##

# If record 'd' does not exist,
# if record 'd' exists but has no field 'value', or
# if record 'd' exists and has field 'value' but is undefined:
if (!$hash{'d'} || !defined($hash{'d'}{'value'}))

##</code><code>##

# If record 'd' does not exist,
# if record 'd' exists but has no field 'value',
# if record 'd' exists and has field 'value' but is undefined,
# if record 'd' exists and has field 'value' but is 0,
# if record 'd' exists and has field 'value' but is "0", or
# if record 'd' exists and has field 'value' but is '':
if (!$hash{'d'} || !$hash{'d'}{'value'})