This is a similar approach to GrandFather's and BrowserUK's but uses split and map to break the text up. The regular expression allows for a split between punctuation and a lower-case letter to cope with "TRUE! nervous" which you break over two lines. Here it is

use strict;
use warnings;

my $rxBoundary = qr
   {(?xms)
      (?:
         (?<=[a-z])(?=[A-Z])
         |
         (?<=[.!?])(?=\s?[A-Za-z])
      )
   };

my $text = <DATA>;
chomp $text;

print
   map {qq{$_\n\n}}
   split m{$rxBoundary}, $text;

__END__
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.
[download]

and the output is

The Tell-Tale Heart

Edgar Allen Poe

TRUE!

 nervous, very, very dreadfully nervous I had been and am; but why WIL
+L 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 whole story
+.
[download]

I hope this is of use.

Cheers,

JohnGG