TIMTOWTDI

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11123325
use warnings;

print do { local $/; <DATA> } =~
  s/AAAAAA\n\K.*?(?=BBBBBB)/<<'END_OF_REPLACE'/gesr =~
f11
f12
f13
END_OF_REPLACE
  s/CCCCCC\n\K.*?(?=DDDDDD)/<<'END_OF_REPLACE'/gesr;
f21
f22
END_OF_REPLACE

__DATA__
aaa
aaa
AAAAAA
ccc
ddd
BBBBBB
111
222
333
CCCCCC
444
555
666
DDDDDD
777
888
999
[download]

Outputs:

aaa
aaa
AAAAAA
f11
f12
f13
BBBBBB
111
222
333
CCCCCC
f21
f22
DDDDDD
777
888
999
[download]