#!/usr/bin/env perl
use strict;
use warnings;

my $book = do {local $/; <DATA>}; # slurp the book

# Split book into words (delimited by \b{wb}), sequences of newlines,
# and sequences of anything else.

while ($book =~
       /(
            ( [^\w\n]+ )
        |
            ( \b{wb}.+?\b{wb} )
        |
            ( \n+ )
        )
       /xg)    {
    show($1);
}
print "\n";

# show(): make spaces and newlines visible
sub show {
    my $str = shift;
    $str =~ tr/\n/$/;
    $str =~ tr/ /_/;
    print "{$str}\n";
}
__DATA__
--First paragraph--
Second one's followed by only one newline. "Hello," she said, "How's t
+ricks?"

Third paragraph doesn't end with any punctuation ... and the splitting
+ works

4th one is separated by two newlines.

         The End.
[download]

{--}
{First}
{_}
{paragraph}
{--}
{$}
{Second}
{_}
{one's}
{_}
{followed}
{_}
{by}
{_}
{only}
{_}
{one}
{_}
{newline}
{._"}
{Hello}
{,"_}
{she}
{_}
{said}
{,_"}
{How's}
{_}
{tricks}
{?"}
{$$}
{Third}
{_}
{paragraph}
{_}
{doesn't}
{_}
{end}
{_}
{with}
{_}
{any}
{_}
{punctuation}
{_..._}
{and}
{_}
{the}
{_}
{splitting}
{_}
{works}
{$$}
{4th}
{_}
{one}
{_}
{is}
{_}
{separated}
{_}
{by}
{_}
{two}
{_}
{newlines}
{.}
{$$}
{_________}
{The}
{_}
{End}
{.}
{$}
[download]