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";