#!/usr/bin/perl -w

package Tie::Hash::Test;
use Tie::Hash;
use vars qw(@ISA);
@ISA = 'Tie::StdHash';
sub TIEHASH {
  my $class = shift;

  my %self;
  @self{@_} = (undef) x @_;

  bless \%self, $class;
}

package main;

tie my %hash, 'Tie::Hash::Test', 'one', 'two', 'three';

print join(':', keys %hash), "\n";