OK perldoc is for sure a role model.

so no parens, no sigils, all parameters are scalars, a final list is denoted as LIST (UPDATE: compare splice))

But is there a perldoc showing the use of named parameters?

What about passing references?

Maybe the Conway notation with a trailing _REF e.g.  RECTANGLE_REF

Don't you think that something like \@rectangle might be better?

My ideal is to have a notation that can be used right away (and automatically) for documentation and for testing.

Maybe a dry discussion about DRY principles ;-)