Perl Module Basics

This should definitely look familiar by now it's the same basic constructor pattern 

you've seen earlier in the chapter.

The rest of the methods are more interesting. FETCH() is similar to the methods 

in a tied scalar, but it receives an extra parameter the key that's being requested. 

The implementation here is very simple:

# method called when an entry is read from the hash

sub FETCH {

   my ($self, $key) = @_;

   # check cache and return if found

   return $self >{cache}{$key} if exists $self >{cache}{$key};

   # make lookup using nameserver provided to TIEHASH

   my $result = _do_dns_lookup($self >{nameserver}, $key);

   # cache result and reverse mapping

   $self >{cache}{$key} = $result;

   $self >{cache}{$result} = $key;

   # return result

   return $result;

}

It's debatable whether BOA::Network::DNS should even provide a STORE()

method DNS entries are generally considered to be read only! However, for the 

sake of completeness, let's provide one. STORE() takes two parameters, the key and 

the value to be set for that key:

# called when an entry is written to the hash

sub STORE {

   my ($self, $key, $value) = @_;

   # store the value in the cache, forward and reverse

   $self >{cache}{$key} = $value;

   $self >{cache}{$value} = $key;

   # return the value stored so that chained assignment works

   return $value;

}

Perl's hashes distinguish between an entry containing undef and an entry that 

doesn't exist at all. The defined() operator simply calls FETCH() on tied hashes, but 

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