Depending on the rest of the code, you might be able to stuff the

 my $id = $self->query->param('id');
 $self->param("object") = $self->get_obj($id) if $id;
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in the setup() method.

Then you "only" need to do

sub foo {
   my $self = shift;
   my $object = $self->param("object") or return $self->get_id();
   # ... the rest 

}
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If you *always* need that object, you might do

sub cgiapp_prerun {
  # ...
  my $id = $self->query->param('id');
  if (!$id) {
     return $self->prerun_mode('get_id');
  }
  $self->param("object") = $self->get_obj($id);
# ...
}
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which will automatically run the get_id mode unless an id is specified, so all other modes can be certain the object is available in $self->param("object").