The following does the same thing. Notice the use of MCE::Loop::init for specifying MCE options. One may pass more then 2 arguments to MCE->gather with this demonstration.

#!/usr/bin/perl

use warnings;
use strict;
use POSIX qw(strftime);
use Time::Piece;
use Cwd qw( abs_path );
use File::Basename;
use File::Basename qw( dirname );
use LWP::Simple;
use MCE::Loop;
use Date::Calc qw (Delta_Days);
use Data::Dumper;

my $domains = dirname(abs_path($0)).'/domains.txt';
my $flag = 0;
my $date = strftime "%Y-%m-%d", localtime;
my %month = ( 'jan'=>'01','feb'=>'02','mar'=>'03','apr'=>'04','may'=>'
+05','jun'=>'06','jul'=>'07','aug'=>'08','sep'=>'09','oct'=>'10','nov'
+=>'11','dec'=>'12' );

my %domainList;

MCE::Loop::init {
   chunk_size => 1, max_workers => 5,
   gather => sub {
      my ($domainName, $diff) = @_;
      $domainList{ $domainName } = $diff;
   }
};

mce_loop_f {
    my $domainName = $_;
    chomp $domainName;
    print $domainName."\n";
    my $expDate = `jwhois -n -h whois.crsnic.net $domainName | grep Ex
+piration | awk '{print \$3}'`;
    print $expDate;
    my $diff = &dateDiff($expDate);

    if ($diff < 28 and $diff > 14) { $flag = 1; }
    elsif ($diff <= 14) { $flag = 2; }

    MCE->gather($domainName, $diff);
} $domains;

MCE::Loop::finish;

print Dumper(\%domainList);

...
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