perl -ne 'print if (/MYLARGEREGEX_HERE/../END_OF_BLOCK/)' inputfile > outputfile

##</code><code>##

#!perl

use strict;
use warnings;

open my $fh_big_file, '<', $ARGV[0] || die; #first argument must be the input file
open my $fh_regex, '<', $ARGV[1] || die; # second argument points to the file containing the regex
my $regex = <$fh_regex>;


while(<$fh_big_file>) {
  print if (/$regex/../^END_OF_BLOCK/);
}

##</code><code>##

while(<$fh_big_file>) {
  print if (/MY_HUGE_REGEX_JUST_PLAIN/../^END_OF_BLOCK/);
}

##</code><code>##

while(<$fh_big_file>) {
  print if (/$regex/o .. /^END_OF_BLOCK/);
}