It defaults to 5.14.2.

If you use 5.10, you get most of the new features of 5.10, regardless whether you use a use 5.10 or not. For give/when, state and say, you need to enable them using the feature pragma -- which will be enabled for you if you do a use 5.10.

This is the same with 5.14.2. You get most of the features of 5.14.2, except then ones you need to enable explicit. Writing use 5.14.2 implies the features give, state, say, and unicode_strings are enabled, and that strict is turned on.