our $dispatch_map = {
  xml => 'My::XML',
  json => 'My::JSON',
};

sub new {
 ...

  die unless exists $dispatch_map->{$args{source}};

  my $class = $dispatch_map->{$args{source}}
  Module::Load::load ($class);

  $self->{engine} = $class; # or $class->new ($self);
}