OCaml is not strictly pure, but it is definitely more strongly functional than Python or Perl. What matters is not that a language completely disallows mutation so much as that it helps you write in a functional style. The functional path is usually the happy path in OCaml, whereas in Ruby mutation and statefulness just come naturally.

The basis of OCaml is functions that transform one thing into another. The basis of Ruby is objects that encapsulate some state and send messages to other objects to get them to change their state.