First, is the problem that it takes too long, or that it uses too much CPU? If it's that it takes too long, you need to profile wall time, not CPU time. If your application does any significant I/O, that's almost certainly the time bottleneck, and you won't see it when profiling CPU time. Until you've done this, forget about method call overhead and inline functions.
Next, stop using fields. The feature you're referring to was called pseudohashes and it was killed a long time ago (5.8 I think) because it caused problems and didn't deliver on the expected improvements. If you want hashes that don't autovivify keys, use Hash::Util::lock_keys(). If you're not on 5.8 yet, it would be a good move to get there.