The following regex using lookback and look ahead assertions gets pretty close. However I don't think there is a way to distinguish between the two places where "mad?" is used except by explicit special cases.

use strict;
use warnings;

my $str = <<DATA;
The Tell-Tale HeartEdgar Allen PoeTRUE! nervous, very, very dreadfully
+ nervous I had been and am; but why WILL you say that I am mad? The d
+isease had sharpened my senses, not destroyed, not dulled them. Above
+ all was the sense of hearing acute. I heard all things in the heaven
+ and in the earth. I heard many things in hell. How then am I mad? He
+arken! and observe how healthily, how calmly, I can tell you the whol
+e story.
DATA

$str =~ s/(?<=[a-z])(?=[A-Z])|(?<=[?!.] )(?![a-z])/\n/g;

print $str;
[download]

Prints:

The Tell-Tale Heart
Edgar Allen Poe
TRUE! nervous, very, very dreadfully nervous I had been and am; but wh
+y WILL you say that I am mad? 
The disease had sharpened my senses, not destroyed, not dulled them. 
Above all was the sense of hearing acute. 
I heard all things in the heaven and in the earth. 
I heard many things in hell. 
How then am I mad? 
Hearken! and observe how healthily, how calmly, I can tell you the who
+le story.
[download]