title: How can I determine if a Perl hash contains a key mapping to an undefined value?
body: <p>I need to determine if a Perl hash has a given key, but that key will be mapped to an undef value.  Specifically, the motivation for this is seeing if boolean flags while using <code>getopt()</code> with a hash reference passed into it.  I've already searched both this site and google, and <code>exists()</code> and <code>defined()</code> don't seem to be applicable for the situation, they just see if the value for a given key is undefined, they don't check if the hash actually has the key.  If I  an RTFM here, please point me to the manual that explains this.</p>