use MCE::Simple -strict, (
    spawn_limit => 2,  # MCE::Child processes
    max_workers => 2,  # number of MCE workers
    init_relay  => 1,  # define to enable MCE::relay 
);

# Run MCE in the foreground.

mce_foreach my $i (10..18) {
    MCE::relay {
        # output orderly
        say "$$: $i";
    };
}

# Run MCE in the background.

spawn sub {
    mce_foreach my $i (20..28) {
        MCE::relay {
            say "$$: $i";
        };
    }
};

sync;

# Spin a worker per each input element.
# Up to "spawn_limit" will run simultaneously.
# Blocking otherwise, until next availability.

spawn my $i (30..38) {
    say "$$: $i";
}

sync;

##</code><code>##

60264: 10  # alternating PIDs
60265: 11  # two MCE workers
60264: 12
60265: 13
60264: 14
60265: 15
60264: 16
60265: 17
60264: 18

60267: 20  # alternating PIDs
60268: 21  # two MCE workers
60267: 22
60268: 23
60267: 24
60268: 25
60267: 26
60268: 27
60267: 28

60269: 30  # spawn keyword
60270: 31  # unique PIDs
60271: 32
60272: 33
60273: 34
60274: 35
60275: 36
60276: 37
60277: 38

##</code><code>##

spawn my $i (30..38) {
    say "$$: $i";
}

# filtered/transposed to this, without altering line no.

foreach my $i (30..38) { MCE::Simple::_spawn_a('', sub {
    say "$$: $i";
}); }