i.e. I expect $h{x} to hold a hash

That's the clue - you expect that, but it isn't. You look at the box, and you are dereferencing that undef value explicitly as a hash. If you access keys/values of an undef value, your dereference happens implicitly and the hashref is created (autovivification):

use strict;
my $f;
$f->{foo} = 'bar'; # okay
my ($h, %new);
%new = %$h;        # not okay
[download]

Said otherwise, if you dereference something, you state that it is a reference, which is different to autovivication of an undef value into something.

Clearer?

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

To me it looks like I'm explictly dereferencing (or casting) undef in both cases. Don't you interpret %{VAL} as an explicit dereferencing of VAL?

Of course %{VAL} is an explicit dereferencing of VAL. But assignment to a previously undefined hash reference is not.

My previous example can be thought of and written as (and is in fact)

use strict;
my $f;
($f = {})->{foo} = 'bar';
my %new;
%new = %{undef()};    # not okay
[download]

I hope that clarifies things. Read perlref - the chapter about symbolic references. By saying %{''} you are creating a symbolic reference to the symbol table entry *{''}. Typeglobs can be empty strings, but not undef, so you cannot create a symbol reference to an undef value:

$\ = $/;
$h = '';
${''}{foo} = 'bar';
print "\$h->{foo}: ", $h->{foo};
print "\$h: '", $h, "'";

print *{''}{HASH};
print *{$h}{HASH};

$h = undef;
print *{$h}{HASH};
__END__
$h->{foo}: bar
$h: ''
HASH(0x885d7a4)
HASH(0x885d7a4)
Can't use an undefined value as a symbol reference at - line 11.
[download]

The construct %{$h} being a symbolic reference:

${''}{foo} = 'bar';
$h = '';
%new = %{$h};
print "$_ => $new{$_}\n" for keys %new;
__END__
foo => bar
[download]

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}