How to find all Keys in a hash have a value in Perl

Question

How do you determine if all hash keys have some value ?

Answer 1

From perldoc -f exists:

               print "Exists\n"    if exists $hash{$key};
               print "Defined\n"   if defined $hash{$key};
               print "True\n"      if $hash{$key};

               print "Exists\n"    if exists $array[$index];
               print "Defined\n"   if defined $array[$index];
               print "True\n"      if $array[$index];

A hash or array element can be true only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.