I've found that when working on something like this it's typically a good idea to create some sort of record for yourself up front. Perl has a marvelous afinity for producing such records out of heaps of data quickly. To embilish your pseudo code a bit:

#!/usr/bin/perl -w
use strict;

my(@fields) = qw/field1 field2 .../ ; # all 21 of your fields CAREFULL
+Y checked

open(FH, "<control.txt") or die "yada... $!";
my($rec) ;
while (<FH>) {
    
   @$rec{@fields} = split /:/ ; # hash slice into an auto-vivified has
+h-reference (instant record)

   $recs{"$rec->{field13}$rec->{field21}"} = $rec ;
}

##
## Individual records can be manipulated by field name
##

$rec{keyOfInterest}->{fieldOfInterest} = NewValue ;

##
## to convert a rec back into a string:
##
my($str) = join ":", @$rec{@fields} ;
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