Those are global. In that "some" hashes are used to upload to DB & all the hashes used to write an xlsx sheet

So, presumably that post file-processing code runs where I've added comments below?

foreach my $linkarray (1 .. 2) 
{
    $pm->start and next LINKS; # do the fork
    
    if ($first == 1)
    {
        my @cdr_list1 = `ls $cdr_directory/SMSBcastCDR_*_$bcat_cdrdate
+\_*.log`;
        print "cdrs_file1 = @cdr_list1\n";
        SMSBcastCDR(@cdr_list1);
        $first++;
                ## Update DB and produce xlsx sheet from global hashes
+ and arrays here????
    }
    if ($first == 2)
    {
                my @smsc_cdr_list=`ls $smscdr_directory/SMSCDR_P*_$cdr
+date*.log`;
        SMSCDR(@smsc_cdr_list);
                ## Update DB and produce xlsx sheet from global hashes
+ and arrays here????
    }
    $pm->finish; # do the exit in the child process
}
$pm->wait_all_children;
[download]

I have another suggestion: Instead of waiting for all your files to finish being written at midnight before starting your processing; run your process(es) to constantly monitor their respective directories throughout the day and process the files as they arrive.

That way, when the final file arrives there'll only be a tiny amount of processing left to do and your stats will be available very shortly after.