The higher-level 'd' key is autovivified by checking for a lower-level key. Check existance of the higher-level key before doing anything with the lower level.

if (exists $hash{'d'} and !$hash{'d'}->{'value'}) {
    print "That key doesn't exist. Valid keys are: ";
    print join (", ", keys %hash);
}
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This is a well-known gotcha.