use strict;

my $count = 0;
open (IN, "input.txt");
while (<IN>) {
    if (/START/) {
	$count = 1;
    }
    elsif (/END/) {
	$count = 0;
    }
    elsif ($count) {
	print;
    }
}
close IN;

##</code><code>##


These are the first

set of lines

which are to be extracted


These are the second

set of lines

which are to be extracted
##</code><code>##

elsif ($count && $_ !~ /^$/)