What is the reason for the implicit block in Ruby invocations?

Is it purely syntactic sugar, to allow things to look 'built-in'

   my_func bar, baz  { ... block ... }

Has anyone tried , or knows if it is even feasible, to have an
extensible syntax in a dynamically-typed language like Ruby? Something
where a class can define not just its method body, but also its own
combination of mix-fix invocation syntax, along the lines of:

  http://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-121.html

Still a newbie...