#! perl -slw
use strict;
use threads;

sub traverse {
    my $href = shift;

    $_->join for map async{
        print "Chapter $_ started";
        my $v = $href->{ $_ };
        ref( $v ) eq 'HASH' and  traverse( $v );
        sleep 1;
        print "Chapter $_ finished";
    }, keys %$href;
}

my %document = (
    '1' => { '1.1' => 'xxx', '1.2' => { '1.2.1' => 'xxx', '1.2.2' => 'xxx' } },
    '2' => { '2.1' => { '2.1.1' => 'xxx' } }
);

traverse( \%document );

__END__
C:\test>1169475
Chapter 1 started
Chapter 2 started
Chapter 1.1 started
Chapter 1.2 started
Chapter 2.1 started
Chapter 1.2.2 started
Chapter 1.2.1 started
Chapter 2.1.1 started
Chapter 1.1 finished
Chapter 1.2.2 finished
Chapter 1.2.1 finished
Chapter 2.1.1 finished
Chapter 1.2 finished
Chapter 2.1 finished
Chapter 1 finished
Chapter 2 finished