The following will give you some information about the hashing:
print scalar %hash;
See perldata for information: search for "bucket":
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.
However, this probably won't help you because you have no control over the hashing mechanism.
Ultimately, the only way for you to answer your question is to use Benchmark.