#!/usr/bin/perl
use warnings;
use strict;

use Time::HiRes qw{ time };

my $start = time;
while (<>) {
    printf '%3.7f | %s', time - $start, $_;
}
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Test run:

for i in {1..10} ; do sleep .2 ; echo $i ; done | prepend.pl
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If you want to overwrite a line, don't output "\n" at its end, but use "\r" instead, it will move the cursor to the beginning of the same line.

Update: See Time::HiRes , printf, and Quote and Quote like Operators.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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