print for split m[
    (?<! Cf )
    (?: (?<! \w\d\d\d ) | (?<! \w\d\d ) | (?<! \w\d ) )
    \.
    (?! [ ^\] ]+ \] )
]x, $s;;

The fox did it[ at 12.23 ] well, Cf. 23 A423.23
 The swallow was even better,