Errata

It looks like that plain '-' cannot be used for open as the answer presents. The following, simplified code would die with "No such file or directory" message:

my $file = '-';
open my $fh, '<', $file or die "Can't open file: $!";

The basic remedy would be exclude '-' from @ARGV and handle it separately by STDIN filehandle, but there may be some better way.

This is minor issue. The each on array in covered in Learning Perl (in arrays chapter), at least starting from 6th edition:

"Perl 5.12 offers another interesting way to handle this, although we didn’t cover it in Learning Perl."

Minor nitpick. The facilities used in this one-liner are covered in Chapter 9 (at the end), especially the -n switch, which means while(<>) with empty body.

The given solution does not compile, with post mortem as:

Global symbol "$wanted_sub" requires explicit package name at my_program line 22.
Global symbol "$hashes_sub" requires explicit package name at my_program line 23.
Missing right curly or square bracket at my_program line 34, at end of line
syntax error at my_program line 34, at EOF
Execution of my_program aborted due to compilation errors.

I am not sure what was the intent of wanted subroutine inside the do block. This is confusing. The working code may look like:

my( $wanted_sub, $hashes_sub ) = do {
my( $file_count, $directory_count, $link_count );

$file_count->{label} = 'file';
$directory_count->{label} = 'directory';
$link_count->{label} = 'link';

sub {
my $full_path = $File::Find::name;
my $dirname = dirname( $full_path );

$file_count->{dirs}{ $dirname }++ if -f $full_path;
$directory_count->{dirs}{ $dirname }++ if -d $full_path;
$link_count->{dirs}{ $dirname }++ if -l $full_path;
},
sub { ( $file_count, $directory_count, $link_count ) };
};

find( $wanted_sub, @ARGV );

The scope of do encapsulates the hashref, while returning both subs in list context. I believe it is exactly what was intended by author.

This is minor issue. The following line using statement modifier unless:

$sum++ unless /e/;

shall be phrases with if modifier:

$sum++ if /e/;