This won't make a good homework answer, but may help in a real world scenario where is is not viable to read the entire book into memory.

I do realize that RAM is so cheap these days it would need to be one hell of a book to warrant this kind of treatment :D

#! /usr/bin/env perl

use strict;
use warnings;
use autodie;

# This solution will avoid reading the entire
# file into memory. First we will separate the
# input into one file per chapter.


# This will become a filehandle in a bit
my $fh;

# Create a directory for the chapter output
mkdir 'chapters';

# Keep a list of all chapter names
my @chapter_names;

# Diamond operator reads input line by line.
# This can be a file or even STDIN
while (<>) {

    if (/^Chapter/) {
        close $fh if $fh;

        # Open a new file.
        # The filename is the chapter name.
        chomp;
        open $fh, '>', "chapters/$_";
        push @chapter_names, $_;

        next;
    }

    print $fh $_;

}

close $fh;


# Now we can sort the chapter names and
# read them line by line, printing on STDOUT

foreach my $chapter_name (sort @chapter_names) {

    open $fh, '<', "chapters/$chapter_name";

    print "$chapter_name\n";

    while (<$fh>) {
        print $_;
    }

    close $fh;

    unlink "chapters/$chapter_name";

}

rmdir 'chapters';
[download]

Best,

Jim