Another alternative would be to move the hash definition into the function definition, since that is the only place where that is used. This satisfies both the desire to modularize your code, and the need to the have the hash defined everwhere that the msg function is called. In this case the definition for your msg subroutine would be:

sub msg {
    my $msgs = {    '200'   =>  'OK',
                    '400'   =>  'Bad Request',
                    '403'   =>  'Forbidden',
                    '404'   =>  'Not Found',
                    '500'   =>  'Internal Sever Error',
                };

   my $msg_num = $_[0];
   my $msg_txt = "$msg_num $msgs{$msg_num}";
   print "HTTP/1.0 $msg_txt\n";
}
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