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

#!perl

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

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

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