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 );
}

##</code><code>##

Use File::Which qw(which);

sub get_dir_file_list_iter {
    my $dir = shift;

    my $FIND = which 'find';
    my $CMD = qq/$FIND -L "$dir" -type f -print/;

    return (
        sub {
            my $chunk_size = shift // 1;
            my @ary;
            while ( my $fn = <$FH> ) {
                chomp $fn;
                push( @ary, $fn );
                last if @ary == $chunk_size;
            }
            return (@ary);
        }
    );
}

##</code><code>##

use MCE;
use MCE::Candy;

my $iterator = get_dir_file_list_iter($dir);
open my $fh_out, ">, $ fn_out;
my $mce = MCE->new(
    gather      => MCE::Candy::out_iter_fh($fh_out),
    chunk_size  => $iter_file_ct,
    max_workers => $max_workers,
    user_func   => \&parse_files,
)->spawn;
$mce->process($iterator);
$mce->shutdown()
);