Not sure what this has to do with latency. But see perlrequick.

use strict;
use warnings;

my $file = 'testpm.dat';
my $string1 = 'open';
my $string2 = 'close';

open F, $file;
while (<F>) {
# if only there was a way to find one string within another!
}
close F;  

if (I found string1 && I did not find string2) {
  print "Alarm!\n";
}
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