Yes, MCE::Map returns the elements in the corresponding order, similarly to map. Actually, there is not much going on for the manager process - very efficient and not to worry about MCE::Map having to preserve order. I compared to MCE::Loop (not ordered) and sometimes MCE::Map is faster other times MCE::Loop. No clear winner. Here is the version using MCE::Loop.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
use MCE::Util;
use MCE::Loop max_workers => MCE::Util::get_ncpu();

sub collatz {
    my ($start) = @_;
    my @seq = $start;
    push @seq, ($seq[-1] / 2, 3 * $seq[-1] + 1)[$seq[-1] % 2]
        while $seq[-1] != 1;
    return @seq
}

my @sizes = mce_loop_s {
    my @results;
    push @results, [$_, scalar collatz($_)] for @{ $_[1] };
    MCE->gather(@results);
} 1, 1e6;

say "@$_" for reverse +(sort { $b->[1] <=> $a->[1] } @sizes)[0 .. 19];
[download]

MCE::Map   9.392 seconds
MCE::Loop  9.396 seconds
[download]

Regards, Mario