Homework or not, this is too nice a problem to resist to show the beauty of Text::CSV_XS. Sorry.

use 5.18.2;
use warnings;

use Data::Peek;
use Text::CSV_XS qw( csv );

my @f = ("A".."R");
csv (in      => "file.csv",
     headers => \@f,
     filter  => {
         A => sub {
             if ($_ eq "2323" && $_{O} ne "AA") {
                 $_[0]->say (*STDERR, [ "ERROR", @_{@f} ]);
                 return 0;
                 }
             $_ ne "0000" && $_ ne "1111" && $_ ne "2222";
             },
         O => sub {
             if ($_ eq "BC") {
                 $_[0]->say (*STDERR, [ "ERROR", @_{@f} ]);
                 return 0;
                 }
             1;
             }
         },
     on_in   => sub {
         $_{D} =~ tr/+=/ /;
         $_{A} =~ s/^1212$/a/;
         $_{A} =~ s/^2121$/aa/;
         $_{A} =~ s/^3131$/ba/;
         $_{A} =~ m/^(?:6767|9898|5656)$/ && $_{D} =~ m/90 09/ and $_{
+G} = "AA";
         },
     );
[download]

I tested that with

A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
1,2,3,4,5,6,7,8,9,0,a,b,c,d,e,f,g,h
A,B,C,foo + bar = F,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
A,B,C,D,E,F,G,H,I,J,K,L,M,N,BC,P,Q,R
0000,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
1111,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
2222,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
2323,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
2323,B,C,D,E,F,G,H,I,J,K,L,M,N,AA,P,Q,R
1212,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
2121,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
3131,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
6767,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
6767,B,C,D09 09D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
[download]

=>

ERROR,A,B,C,D,E,F,G,H,I,J,K,L,M,N,BC,P,Q,R
ERROR,2323,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
1,2,3,4,5,6,7,8,9,0,a,b,c,d,e,f,g,h
A,B,C,"foo   bar   F",E,F,G,H,I,J,K,L,M,N,O,P,Q,R
2323,B,C,D,E,F,G,H,I,J,K,L,M,N,AA,P,Q,R
a,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
aa,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
ba,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
6767,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R
6767,B,C,"D09 09D",E,F,G,H,I,J,K,L,M,N,O,P,Q,R
[download]