The doc you mentioned is about the variables and preprocessor defines that are available after you perl has been built. The use64bit.U you see there is the name of the metaunit that is used in Configure during configuration to set the variables available through use Config; runtime.

To see the arguments to use on the command-line, this documentation won't help you at all. In fact, https://perldoc.perl.org/ is not showing you those docs at all :/

Worse, I did not see the wanted docs on https://dev.perl.org either.

So, read this instead: INSTALL.