package My

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

   if ($args{source} eq 'xml') {
      require My::XML;
      @ISA = qw/My::XML/;
   } elsif ($args{source} eq 'json') {
      require My::JSON;
      @ISA = qw/My::JSON/;
   }
   $self->_init(); # Versions in both My::XML and My::JSON

   return ($self);
}
# rest of methods
sub method1 { ... }
sub method2 { ... }