sub new {
   my ($class,%args) = @_;

   my $data;
   if ($args{source} eq 'xml') {
      require My::XML;
      # @ISA = qw/My::XML/;
      $data = My::XML->new(%args);
   } elsif ($args{source} eq 'json') {
      require My::JSON;
      # @ISA = qw/My::JSON/;
      $data = My::JSON->new(%args);
   }

   my $self = bless \%{$data}, $class;
   return ($self);
}
# rest of methods
sub method1 { ... }
sub method2 { ... }

1;