G'day chengchl,

Welcome to the Monastery.

Here's a generic solution for your problem. It handles:

• Extraction of any block (i.e. there's no hard-coded or constant block number).
• Extraction of multiple blocks.
• Blocks of lines actually containing (plural) lines.
• Rogue START or END tokens within START-END blocks.
• Specification of wanted blocks in any order.
• Invalid block specifications (e.g. out of range and non-integer identifiers).

In production code, you may want to add some form of validation and sanity checking, such that the function is short-circuited if no valid blocks are specified (which could mean not even having to open the input file).

The following shows the technique (specifically for testing via the command line); you'll need to adapt this to your needs (e.g. change <DATA> to <$fh_r>). I've embedded test data to check all the things I've said it handles; you should create your own test data, which more realistically reflects your actual data, and use that for any proof-of-concept or regression tests.

#!/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
...
[download]

Some example test runs (the script name is pm_1202989_flip_flop_selection.pl):

$ 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 irre
+levant
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
$
[download]

[Side note: As you're new here, you may have been surprised by certain responses. You can safely ignore these; a quick perusal of the "Worst Nodes" page should explain why.]