a($_) while <DATA>;

for (sort keys %h){
	print "$_: my address is $h{$_}\n";
	print "last:  $h{$_}{'a'} >>> $h{$_}{'a'}{'c'}\n";
	print "next:  $h{$_}{'b'} >>> $h{$_}{'b'}{'c'}\n\n";
}

sub a {
		chomp;
		$t = shift;
		$h{$t}{'a'} = $l;
		$h{$t}{'c'} = $t;
		$$l{'b'} = $h{$t};
		$l = $h{$t};
}

__DATA__
a
b
c
d
e


##</code><code>##

a: my address is HASH(0x2b230)
last:   >>>
next:  HASH(0x22e44e8) >>> b

b: my address is HASH(0x22e44e8)
last:  HASH(0x2b230) >>> a
next:  HASH(0x22e45d8) >>> c

c: my address is HASH(0x22e45d8)
last:  HASH(0x22e44e8) >>> b
next:  HASH(0x22ef2a0) >>> d

d: my address is HASH(0x22ef2a0)
last:  HASH(0x22e45d8) >>> c
next:  HASH(0x22ef468) >>> e

e: my address is HASH(0x22ef468)
last:  HASH(0x22ef2a0) >>> d
next:   >>>