The "secret sauce" is in the line such as $myhash->{'mykey'}++. Welcome to what Perl calls "auto-vivification." If a hash-entry does not yet exist, it will be created with the initial value of 1. If it does exist, it will be incremented. So you can write this one statement to "do what you want."

After looping through all the data, iterate through the keys in the hash and look for values greater than 15.