Thanks for all of your input. Here is the code snipped I used:

# Timestamp 12 hours ago
my $twelve_hours_ago = strftime("%Y-%m-%d/%H:%M:%S", (localtime(time-$
+hours*3600)));


# Logfile may have been rotated during the past 12 hours.  Thus we mus
+t open
# any logfile that is less than 12 hours old.
find(\&wanted, "$logfile");

sub wanted {

    # Open loge file if it is less than 12 hours old and the 
    # correct file name

    if ( (stat $_)[9] > time() - (3600 * $hours)
        && m/^log_file/ ) {

        # Debugging lines
        #print "file: ".$_."\n";
        #my $stat = (stat $_)[9];
        #print "stat: ".$stat."\n";
        #my $twelve = time() - (3600 * $hours);
        #print "twelve: ".$twelve."\n";
        # End of debugging lines.

        open LF, ($_) || die "Cannot open $logfile $!";

        while (<LF>){

            # Check for PASS log entry
            next unless (m/log :Monitor: CALLER Status \*PASS\*/);

            # Check timestamp
            $date = $1 if (m/^([^.]+)/);   # Grab the date up to secon
+ds
            next unless $date gt $twelve_hours_ago;
            #print "date: ".$date."\n"; # For debugging.

            # Count number for passes.
            $passcount++;

            # Save log line
            push @passlogs, $_;
            }
    }
}
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