Re-reading OP again, i think i understood this time ... I believe (untested) that this does the same task:

sub fold {
  my $self = shift;
  my $array = $self->{data};
  return unless $array && @$array;

  my %groups = map { $_ => 1 } @{$self->{groups};
  my @keys = grep { $groups{$_} } qw/ date campaign /;
  my %h;
  foreach my $el ( @$array ){
    my $k = join ":", @{$_}{@keys};         #  This only works if the 
+date & campaign values do not contain ':'
    if( exists $h{$k} ) {
      $h{$k}->{views} += $el->{views};
      $h{$k}->{clicks} += $el->{clicks};
    }else{
      $h{$k} = $el;
    }
  }
  @$array = sort { fold_sort($groups{campaign},$groups{date}) } values
+ %h;
}
[download]

Note that the basic approach is to hash up on the commonality (date and/or campaign) to combine everything, then take those values and sort for final result.