Your code works, except: (1) You are just moving the decimal so, divide by 1,000 instead of 1024, (2) you should round up, I used the ceil() function for that.

Here is a "hack it with regex" approach:
This doesn't exactly replicate your formatting. But to my eye, it looks "close enough".
I just started adding one regex at a time until I got something "close to desired".
Mileage varies. I used "just a complicated enough" regex for your example. I didn't consider other examples.
Again, mileage varies. But maybe you will see something useful in this hack.
Also, I considered running 5 regexes per line to be of no performance consequence at all.

ADDED: One complication that I did see was, what if Name like "DBMS-db2pb13345" appears? A more complex regex would be needed to skip that. Also from your problem statement, it sounds like sometimes this might need to go KB->GB instead of KB->MB? I'm not sure that that would mean. "Commifiying" the numbers adds a lot of readability. Your sub for that is fine and right out of Perl Cookbook! 

use strict;
use warnings;
use POSIX 'ceil';

#-- Format memory in MB from db2pd -dbptnmem
#-- Run as db2pd -dbptnmem | perl -nl db2mem.pl
#-- or  perl -nl db2mem.pl db2mem.PB1

sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text;
}

while (<DATA>){

   print, next if ($.==1); #don't process first line
   
   s/KB/MB/g;                              # all KB->MB
   s|(\d{2,})|commify(ceil($1/1000));|ge;  # move decimal point
   s| (\d+,)|$1|g;                         # allow for one comma in al
+ignment
   s|   (Mem Used)|$1|;                    # adjust columns to left 3 
+spaces
   s|===||;                                # shorten separator 3 colum
+ns
   print;
}   

=OUTPUT
Database Member 0 -- Active -- Up 26 days 13:26:26 -- Date 2020-07-10-
+10.11.32.332869

Database Member Memory Controller Statistics

Controller Automatic: N
Controller License Limit: N
Controller Limit Enforced: Y

Memory Limit:        26,598 MB
Current usage:       25,769 MB
HWM usage:           26,004 MB
Cached memory:       3,353 MB

Individual Memory Consumers:

Name          Mem Used (MB) HWM Used (MB) Cached (MB)
=====================================================
APPL-PB1                197        602       39
DBMS-db2pb1             211        211        5
FMP_RESOURCES            23         23       21
PRIVATE               1,378      1,425      844
DB-PB1               23,962     24,334    2,445

=cut

__DATA__
Database Member 0 -- Active -- Up 26 days 13:26:26 -- Date 2020-07-10-
+10.11.32.332869

Database Member Memory Controller Statistics

Controller Automatic: N
Controller License Limit: N
Controller Limit Enforced: Y

Memory Limit:         26597404 KB
Current usage:        25768448 KB
HWM usage:            26003008 KB
Cached memory:        3352576 KB

Individual Memory Consumers:

Name             Mem Used (KB) HWM Used (KB) Cached (KB)
========================================================
APPL-PB1                196352        601792       38272
DBMS-db2pb1             210752        210752        4736
FMP_RESOURCES            22528         22528       20736
PRIVATE                1377280       1424640      843968
DB-PB1                23961536      24333568     2444864

[download]

In reply to Re: Memeory usage output KB to MB conversion by Marshall
in thread Memeory usage output KB to MB conversion by Perlistan

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