#!/usr/bin/env perl

use strict;
use warnings;

my %print_block = map { $_ => 1 } @ARGV;

my $found_block = 0;

while (<DATA>) {
    next unless /^START$/ .. /^END$/;
    ++$found_block, next if /^START$/;
    next if /^END$/;
    print if $print_block{$found_block};
}

__DATA__
...
line BEFORE any wanted blocks
...
START
block A line 1
block A line 2 with rogue END token
block A line 3
block A line 4 with rogue START token
block A line 5
END
...
line BETWENN any wanted blocks
...
START
block B line 1
block B line 2 with rogue START token
block B line 3
block B line 4 with rogue END token
block B line 5
END
...
line BETWENN any wanted blocks
...
START
block C line 1
block C line 2 with rogue END token
block C line 3
block C line 4 with rogue START token
block C line 5
END
...
line BETWENN any wanted blocks
...
START
block D line 1
block D line 2 with rogue START token
block D line 3
block D line 4 with rogue END token
block D line 5
END
...
line AFTER any wanted blocks
...

##</code><code>##

$ pm_1202989_flip_flop_selection.pl
$ pm_1202989_flip_flop_selection.pl 99
$ pm_1202989_flip_flop_selection.pl A B C
$ pm_1202989_flip_flop_selection.pl 1
block A line 1
block A line 2 with rogue END token
block A line 3
block A line 4 with rogue START token
block A line 5
$ pm_1202989_flip_flop_selection.pl 1 4
block A line 1
block A line 2 with rogue END token
block A line 3
block A line 4 with rogue START token
block A line 5
block D line 1
block D line 2 with rogue START token
block D line 3
block D line 4 with rogue END token
block D line 5
$ pm_1202989_flip_flop_selection.pl 3 4 2 # NOTE: specified order irrelevant
block B line 1
block B line 2 with rogue START token
block B line 3
block B line 4 with rogue END token
block B line 5
block C line 1
block C line 2 with rogue END token
block C line 3
block C line 4 with rogue START token
block C line 5
block D line 1
block D line 2 with rogue START token
block D line 3
block D line 4 with rogue END token
block D line 5
$