use strict;
use warnings;

@_ = <DATA>;
s/^\s+|\s+$//g for @_;
$_ = join "\n", @_;
##### END OF LINE MARKERS
s/([^\.\?!\n])\n+/$1 /g;
##### ADD TO AS NECESSARY
print;

__DATA__
This line breaks

in the middle.

This one doesn't.
Neither does this!
Or this?

I hope.