You've been bitten by auto-vivification. It means that since $hash{'d'} is undefined and you're asking it to be a hash reference, Perl creates an anonymous hash for you.

You probably want

# If record 'd' does not exist:
if (!$hash{'d'})
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Not as likely, but you might even want

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

# 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'}))
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However, the equivalent to your code would be

# 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'})
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Thanks for reducing the problem to a its minimum form.