#!/usr/bin/env perl

use 5.010;

use strict;
use warnings;

my %marker = (
    "AAAAAA\n" => {
        end     => "BBBBBB\n",
        repl    => <<'EOD',
f11
f12
f13
EOD
    },
    "CCCCCC\n" => {
        end     => "DDDDDD\n",
        repl     => <<'EOD',
f21
f22
EOD
    },
);

while ( <DATA> ) {
    state $section;
    warn "$. $_";
    if ( $section ) {
        if ( $_ eq $section->{end} ) {
            warn "========= End of the fragment detected at $.\n";
            print $section->{repl};
            $section = undef;
        }
    } elsif ( $section = $marker{$_} ) {
        warn "========= Start of the fragment detacted at $.\n";
    } else {
        print;
    }
}

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