Instead, package users should use the state_name_for subroutine to access it. This will help coders avoid this pitfall:

if( $MyConfig::state_name_for{AL} = 'Alaska' ){
   print  "oops! we just assigned Alaska to the AL key!\n";
}
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But your state_name_for is defined

sub state_name_for{
  return \%state_name_for;
}
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That returns a reference to the hash %state_name_for and exposes it to the user just like accessing the hash directly. To protect the hash, put something like this in MyConfig:

sub get_state_name { $state_name_for{ +shift} }
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and keep %state_name_for a private lexical in MyConfig. Now the user can't change the hash content, accidentally or deliberately. They still can query the hash the way it is intended.

Anno