A regex to match a time within the past 12 hours is doable, but it's probably not nearly as efficient as doing two passes, one to collect the time (if it's a PASS entry) and one to compare that time. But because I can't back away from a challenge:

use Time::Local;

while (<$file>) {
  if (m{
    ^
    (\d\d\d\d) - (\d\d) - (\d\d)  # capture y/m/d to $1,$2,$3
    /
    (\d\d) : (\d\d) : (\d\d)      # capture h/m/s to $4,$5,$6
    (?(?{
      timelocal($6,$5,$4,$3,$2-1,$1-1900) < (time - 12*60*60)
    })(?!))  # fail if timestamp is older than 12 hours
    .* \*PASS\*
  }x) {
    # it's an ok line!
  }
}
[download]

Untested, but should work fine.