use File::Map qq(map_file);
use MCE;
use MCE::Candy;
use Path::Iterator::Rule;

my $rule = Path::Iterator::Rule->new->file->name(qr/[.](html)$/);
my $iterator = $rule->iter_fast("topdir");

open my $fh_out, ">:utf8", $self->fn_out;

my $mce = MCE->new(
    gather      => MCE::Candy::out_iter_fh($fh_out),
    max_workers => 'auto',
    user_func   => \&parse_file,
)->spawn;
$mce->process($iterator);

sub parse_file {
    my ( $mce, $chunk_ref, $chunk_id ) = @_;
    map_file my $text, $chunk_ref->[0], '<';
    my (@posts) = $text =~ m/
        \<\!--XXXX:\ (\d+)--\>\<\!--YYYY:\ (\d+).+?
        (?:<\!--AAAAA--\>(.*?)\<\!--\/AAAAA--\>|)
        \<\!--BBBB--\>(.*?)\<\!--\/BBBB--\>.+?
        \<\!--CCCC--\>(.+?)\<\!--\/CCCC--\>
    /msgx;

    …  Do some stuff with posts and place 
       results in multiline string $output

$mce->gather( $chunk_id, $output );
}