If I'm understanding you correctly, adding sort when retrieving the keys of the inner hash like you do in the outer hash should do what you desire:

sub printIndex {
    for my $book ( sort keys %ibIndex ) {
        
        # notice the use of sort in the next line

        for my $keyword ( sort keys( %{ $ibIndex{ $book } } ) {
            my $pages = join( ', ', @{ $ibIndex{ $book }->{ $keyword }
+ } );
        }
    }
}
[download]