use strict;
use warnings;
use feature qw(say);

use MCE::Loop chunk_size => 1, max_workers => 4;

my @files  = glob '*.gz';

my %result = mce_loop {
    my ($mce, $chunk_ref, $chunk_id) = @_;

    ## $file = $_; same thing when chunk_size => 1

    my $file = $chunk_ref->[0];

    ## http://www.zlib.net/pigz/
    ## For pigz, we want -p1 to run on one core only.
    ## open my $fh, '-|', 'pigz', '-dc', '-p1', $file or do { ... }

    open my $fh, '-|', 'gzip', '-dc', $file or do {
        warn "open error ($file): $!\n";
        MCE->next();
    };

    my $count = 0;

    while ( my $line = <$fh> ) {
        $count++;  # simulate filtering or processing
    }

    close $fh;

    ## Send output to the manager process.
    ## Ensures workers do not garble STDOUT.

    MCE->say("$file: $count lines");

    ## Gather key-value pair.

    MCE->gather($file, $count);

} @files;

## Workers may persist after running. Request workers to exit.

MCE::Loop->finish();

## Ditto, same output using gathered data.

for my $file (@files) {
    say "$file: ", $result{$file}, " lines";
}