package MY::Constants;

my %constants = (
  FRED => 2,
  DINO => 4,
  WILMA => "bb-b-b-b-uck!",
  LARGE => 1e38,
);

sub import {
  my $package = shift; 
  for (@_) {
    unless (exists $constants{$_}) {
      require Carp;
      Carp::croak("Constant $_ invalid");
    }
    {
      no strict 'refs';
      my $full_name = caller()."::$_";
      my $value = $constants{$_}; # for closure
      *$full_name = sub () { $value };
    }
  }
}

'0 but true';