Here is my try: untested. propably you need to sort the keys for your output. And the keys in groups must match the data keys. say date and campain_id in your example.

sub fold {
  my $self = shift;
  my %h;
  my @groups = @{ $self->{groups} };
  for ( @{ $self->{data} } ) {
    push @{ $h{ join $;, @$_{ @groups } } }, $_;
  }

  $self->{data} = [
    map {
      my $x;
      @$x{ @groups } = @{ $h{$_}->[0] }{ @groups };
      for ( @{ $h{$_} } ) {
        $x->{views}  += $_->{views};
        $x->{clicks} += $_->{clicks};
      }
      $x;
      } keys %h
  ];
}
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