... such long chains of method calls tend to be rare in code I write...

In code you write, perhaps.

... to distinguish a method call on the previous term from a method call on $_.

How often does someone expect to chain methods and call a method on $_? Seems to me if you want to change the invocant from whatever the previous expression returned, you ought to make that method explicit, whether by typing it out by hand or terminating the entire statement so that it's not ambiguous at all.