In summary, one can take serial code and enable parallelism with little code.

open my $fh, $domains or die "CRIT: Unable to open $domains: $!\n";

my %domainList;

while( my $domainName = <$fh> ) {
    chomp $domainName;
    my $expDate = `jwhois -n -h whois.crsnic.net $domainName | grep Ex
+piration | awk '{print \$3}'`;
    my $diff = &dateDiff($expDate);
    if ($diff < 28 and $diff > 14) { $flag = 1; }
    elsif ($diff <= 14) { $flag = 2; }
    $domainList{$domainName} = $diff;
}

close $fh;
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Parallelism is possible simply by wrapping MCE around serial code. This does not fork per each input item. Thus, graceful to the OS.

use MCE::Loop chunk_size => 1, max_workers => 5;

my %domainList = mce_loop_f {
    my $domainName = $_;
    chomp $domainName;
    my $expDate = `jwhois -n -h whois.crsnic.net $domainName | grep Ex
+piration | awk '{print \$3}'`;
    my $diff = &dateDiff($expDate);
    if ($diff < 28 and $diff > 14) { $flag = 1; }
    elsif ($diff <= 14) { $flag = 2; }
    MCE->gather($domainName, $diff);
} $domains;
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MCE can also take a file handle as input. However, this is less efficient for large files due to involving the manager process.

use MCE::Loop chunk_size => 1, max_workers => 5;

open my $fh, $domains or die "CRIT: Unable to open $domains: $!\n";

my %domainList = mce_loop_f {
    my $domainName = $_;
    chomp $domainName;
    my $expDate = `jwhois -n -h whois.crsnic.net $domainName | grep Ex
+piration | awk '{print \$3}'`;
    my $diff = &dateDiff($expDate);
    if ($diff < 28 and $diff > 14) { $flag = 1; }
    elsif ($diff <= 14) { $flag = 2; }
    MCE->gather($domainName, $diff);
} $fh;

close $fh;
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