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

##</code><code>##

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

##</code><code>##

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