my $string = "Hello Mr. Jack! How are you? Everything is OK. That's fine!";

my @sentances = split(/
(?<=\.|\!|\?)   # true if in the left side is any of [.!?]
                # - end of a sentence

(?<!Mr\.|Dr\.)  # true if in the left side is *NOT* any of ("Mr." | "Dr.")
(?<!U\.S\.A\.)  # true if in the left side is *NOT* "U.S.A."
                # - ends with a dot, but is the end of a sentence

\s+             # true if in the current possition is space
                # - space between words

(?=[A-Z])       # true if in the right side is a capital A-Z
                # - start of a new sentence
/x, $string);

print $_,"\n" for @sentances;