I agree that determining that the set of keys is not perverse is good first step in understanding poor hash performance. We assume that you asked the more important question...is the hash is the most effective way to the solve your problem. After that, some degree of optimization can be obtained by pre-sizing the hash. Perl may be reorganizing the hash at one or more points in populating it.

If you can calculate the number of keys Perl provides a way for you to reveal the number of keys you will insert. Use keys(%hash_name) = number to provide that estimate. You may want to use a number smaller or larger than the number of keys depending on how Perl uses this hint. (Read the source or experiment?)