use POSIX qw/strftime/;

# generate a timestamp of 12 hours ago in the same format as the data file
$twelve_hours_ago = strftime("%Y-%m-%d/%H:%M:%S",localtime(time-12*3600));
  
while (<FH>) {
   next unless /PASS/;                   # skip the non-PASS entries
   my ($date) = /^([^.]+)/;              # grab the date up to seconds
   next unless $date gt $twelve_hours_ago;
   # process the interesting records.
}