http://www.perlmonks.org?node_id=451795


in reply to Heisenberg Uncertainty Hash

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'})

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'}))

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'}))

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'})

Thanks for reducing the problem to a its minimum form.