In general I lean towards less nesting. Less nesting makes the code easier to map into a model in my head. A lot of nesting in one place make me question if there should be a subroutine.

I have seen code with CLAIM comments that signal to the reader that something is true at that point in the program. (Well, that the programmer intended it to be and believes it to be true.) See the following example.

sub foo
{
   my($self) = @_;
   my $authentication = $self->authenticate_user();
   if(! defined($authentication)){
      return {status=>0, error=>"Authentication Failed", external_id=>
+undef};
   }

   # CLAIM : the user is authenticated

   my $profile = $self->get_user_profile();
   if(! $profile->is_active()){
      return {status=>0, error=>"User is not active", external_id=>und
+ef};
   }

   # CLAIM : the user profile is active

   my($external_id) = $self->find_external_id($profile);
   if(! defined($external_id)){
        $external_id = $self->send_profile_to_partner($profile);
   }

   # CLAIM : the user profile has been sent to the partner

   return {status=>1, error=>undef, external_id=>$external_id};
}
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