In the URI::File module, it shows

  Unix                  URI
  ----------            ------------------
  foo/bar      <==>     foo/bar
  /foo/bar     <==>     file:/foo/bar
  /foo/bar     <==      file://localhost/foo/bar
  file:         ==>     ./file:
  <undef>      <==      file:/fo%00/bar
  /            <==>     file:/
[download]

that is, one slash only if no host, but doesn't show three slashes for that same case. But, I thought the leading // after the file: meant that the rest of the name uses the slash-separated hiarachy, otherwise is opaque, so one slash isn't right? But that wouldn't explain relative URL syntax.

If we have %HH syntax, why map : to | at all? It's not like anyone types filename urls in that syntax.