package Foo::Config;

use ...

BEGIN {
  use Exporter;
  @EXPORT = qw/ &config /;
# the following is the config data
  %config = (
    dev => {
      A => {foo => 'dev_a'},
      B => {this_is_not_foo => 'not_dev_b'},
      _default => {
        foo => 'default_foo',
        database_env => 'dev_db',
      },
    },
    prod => {
      A => {foo => 'prod_a'},
      B => {this_is_not_prod_foo => 'prod_b'},
      _default => {
        foo => 'default_prod_foo',
        database_env => 'prod_db',
      },
    },
    _default => {
      A => {
        default_across_all_a_environments => 'def',
      },
      _default => {
        the_same_everywhere => 1
      },
    },
# other stuff you want to run in the BEGIN block, i.e. set up the default environment.  One thing i do is get a $environment variable based on the hostname.
} #END BEGIN

sub config($) {
  my $field = shift;
  if (exists $config{$environment}->{$sub_environment}{$field} ) {
    return $config{$environment}->{$sub_environment}{$field};
  };

  if (exists $config{'_default'}->{$sub_environment}{$field} ) {
    return $config{'_default'}->{$sub_environment}{$field};
  };
 
  if (exists $config{$environment}->{'_default'}{$field} ) {
    return $config{$environment}->{'_default'}{$field};
  };       

  if (exists $config{'_default'}->{'_default'}{$field} ) {
    return $config{'_default'}->{'_default'}{$field};
  };
  return undef;
}

##</code><code>##

use Foo::Config;
my $config_var = config('foo');