my $re = qr{
    ^ (\S+ \s+ \S+ \s+ \S+) \s+ # time_stamp: Sep 18 00:00:58
    (\S+) \s+                   # host:       mailgate04
    [[:alpha:]]+/([[:alpha:]]+) # process:    postfix/smtp:
    \S+ \s+                     # PID:        [29259]:
    (.*)                        # rest
    }ox;
while(<$log>) {
   my ($time, $host, $rest) = /$re/o;
   ...
}