Since the time information inside of your files seems to be in a good enough form that you could compare them directly with a string comparison, you could do something like this:

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*360
+0));
  
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.
}
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Update: I had a numeric comparator where clearly a string comparator is warranted and fixed the problem with the 12 hour ago timestamp. Thanks neilwatson, japhy

Update #2: While I'm thinking about it, note that this solution probably breaks at DST boundaries.