This looks like a solution based on ingenious emulation of Perl range operator with the advantage that it can accommodate multiple fragments.

Like in Perl range operator there is a variable this is true within the range and false outside.

Thank you !

Here is a slightly modified variant that I used for testing:

[0]  # cat  test_range2
#!/usr/bin/env perl

use 5.010;

use strict;
use warnings;
my $debug=0;
my %marker = (
    "AAAAAA\n" => {
        end     => "BBBBBB\n",
        repl    => <<'EOD',
f11
f12
f13
EOD
    },
    "CCCCCC\n" => {
        end     => "DDDDDD\n",
        repl     => <<'EOD',
f21
f22
EOD
    },
);

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

}

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

[0]  # perl  test_range2
aaa
aaa
AAAAAA
f11
f12
f13
BBBBBB
111
222
333
CCCCCC
f21
f22
DDDDDD
777
888
999