#!/usr/bin/perl
#timestamp.pl
###
use Time::HiRes qw(gettimeofday);
$|=1;

while (<>) {
        $timestamp = sprintf("%d.%06d", gettimeofday());
        print "$timestamp $_";
        }
exit;


---
touch testfile
tail -f testfile | timestamp.pl

_in another window_
echo 'test' >> testfile
(wait a while)
echo 'test' >> testfile
(wait a few seconds)
echo 'test' >> testfile
(wait a little while)
echo 'test' >> testfile