$s = 'The fox did it[ at 12.23 ] well, Cf. 23 A423.23. The swallow was even better,';;

print for split '(?<!Cf)(?<!\w\d{3})\.(?![^\]]+])', $s;;
The fox did it[ at 12.23 ] well, Cf. 23 A423.23
 The swallow was even better,