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.

I wrote it before and I'm repeating it here, again without proper attribution (anyone here?) nor the certainty of quoting it really correctly: "the value of a hash in scalar context is of no use to a Perl programmer, only to a perl programmer."