I would suspect that the problem is due to the fact that your code has to recompile the regex each time through the loop.

Perhaps using the o modifier would help: 

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

[download]
Update: or rather: 
while(<$fh_big_file>) {
  print if (/MY_HUGE_REGEX_JUST_PLAIN/o .. /^END_OF_BLOCK/);
}

[download]
Update 2: sorry, wrong copy and paste (and thanks to shmem for pointing out): 
while(<$fh_big_file>) {
  print if (/$regex/o .. /^END_OF_BLOCK/);
}

[download]

In reply to Re: Performance of assambled regex by Laurent_R
in thread Performance of assambled regex by Foodeywo

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