#! perl -slw
use strict;

( my $data = do{ local $/; <DATA> } )=~ s[\n\+][]g;

print "'$1'" while $data =~ m[
    ( .+?                        ## Capture everything (minimally)
       (?: [.!?] \s+             ## A terminator followed by whitespace
         | [a-z]                 ## or a lowercase letter
       )    
    ) 
    (?= [A-Z] | \z )             ## followed by a an uppercase letter or EOS
]xg;

__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 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 i
+n hell. How then am I mad? Hearken! and observe how healthily, how calmly, I ca
+n tell you the whole story.