If you evaluate a hash in scalar context, it returns false if the hash is empty. If there are any key/value pairs, it returns true; more precisely, the value returned is a string consisting of the number of used buckets and the number of allocated buckets, separated by a slash. This is pretty much useful only to find out whether Perl's internal hashing algorithm is performing poorly on your data set. For example, you stick 10,000 things in a hash, but evaluating %HASH in scalar context reveals "1/16", which means only one out of sixteen buckets has been touched, and presumably contains all 10,000 of your items. This isn't supposed to happen
It is not useful to assign a hash to a scalar. It is useful to store a reference to a hash in a scalar though, e.g.
my %hash = (foo => 1);
my $ref = \%hash;
This seems like an odd question to ask - are you looking for perl trivia, or trying to solve a problem?