I like the answers I've seen, but I thought you might like to see one that stays very close to the original code, and uses the conditional logic you posted in your Perl comments.

#!/usr/bin/perl -w

use strict;

my %a = qw(
      60622  1
      60516  1
      60201  1
   );

my %b = qw(
      90210  1 
      60622  1
      12345  1
   );

my %c = qw(
      11412  1
      32134  1
      60201  1
   );

open OUTPUT_A, ">a.txt" or die "Can't open OUTPUT_A: $!\n";
open OUTPUT_B, ">b.txt" or die "Can't open OUTPUT_B: $!\n";
open OUTPUT_C, ">c.txt" or die "Can't open OUTPUT_C: $!\n";

while (<DATA>) {
   chomp;
   my($fn, $ln, $id) = split(",", $_);
   
   if (exists $a{$id}) {
      print OUTPUT_A $fn, $ln, $id;
   } elsif (exists $b{$id}) {
      print OUTPUT_B $fn, $ln, $id;
   } elsif (exists $c{$id}) {
      print OUTPUT_C $fn, $ln, $id;
   } 
}

close OUTPUT_A; 
close OUTPUT_B; 
close OUTPUT_C;

__END__
Homer,Simpson,60622
Clark,Kent,90210
Fred,Flintstone,00987
[download]

This code creates an associative array for each output file, and checks to see if each $id value exists as a key in the table.

As to your other comments... most of us go through such frustrations at times. The other monks have given you great counsel about relaxing.

One recommendation I would add for improving your Perl proficiency: check out the various perlfaq sections in the documentation. Especially perlfaq4 about handling different kinds of data.

You could also browse the Monastery's "Categorized Q&A" and "Tutorials" sections.