while (<LOGFILE>) {
  next unless /\b(\d\d:\d\d:\d\d)\b/;

  if (($1 ge '08:45:00' && $1 le '12:00:00')
   or ($1 ge '14:00:00' && $1 le '17:00:00')) {
    # do stuff.
    # line is in $_, time is in $1
  }
}