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
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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
$h: ''
HASH(0x885d7a4)
HASH(0x885d7a4)
Can't use an undefined value as a symbol reference at - line 11.
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The construct %{$h} being a symbolic reference:

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

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