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 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.
DATA

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

print $str;