This seems to solve the issue:

my @fragments = grep length, split /(\b{wb}\w.*?\b{wb}|\n+)/, $book;
[download]

But my knowledge of Unicode and the \b{wb} semantics is rather limited so that may have other issues.

Not sure 'cause that's 'bout words also including non \w characters.

And some of 'em even start on apostrophe ;)

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

Well, specifically for the apostrophe, \b{wb} doesn't seem to take initial ones as part of words. It breaks your sample sentence as follows: {Not} {_} {sure} {_'} {cause} {_} {that's} {_'} {bout} {_} {words} {_} {also} {_} {including}

But I agree with you that there are probably other cases of words (as defined by \b{wb}) that don't start by a character matching \w.

At the end, my conclusion is that the only way to handle the OP problem in a way fully consistent with \w{wb} semantics is to just split using it, and maybe repack non word fragments afterwards:

my $book = "Not sure 'cause that's 'bout words also including ...\n.\n
+_\n\n...";

my @fragments;
my $last_was_symbol;

for (split /\b{wb}/, $book) {
    if (/[\w\n]/) {
        $last_was_symbol = 0;
        push @fragments, $_;
    }
    else {
        if ($last_was_symbol) {
            $fragments[-1] .= $_;
        }
        else {
            push @fragments, $_;
            $last_was_symbol = 1;
        }
    }
}

sub show {
    my $str = shift;
    $str =~ tr/\n/$/;
    $str =~ tr/ /_/;
    print "{$str} ";
}

show $_ for @fragments;
print "\n";
[download]

For my purposes, this is fine. I'm mainly interested in capturing possessives and contractions.