If "/" is the delimiter then the initial m is optional.

I think this statement in Regexp Quote-Like Operators (in the discussion of  m//) is intended, albeit without the best clarity, to refer to the case of implicit matching against $_. In this case, the statements  /foo/; or  m/foo/; or  m{foo}; or  m'foo'; will all operate as intended, but the statements  {foo}; or  'foo'; will obviously not: they are simply null statements.