#!/usr/bin/perl -w
use strict;

my $start = time;    # let's see what time it is

our $gap ||= 1;      # how many seconds between prints

while (1) {          # and from now on
  local $|   = 1;
  my    $now = time;    # see what time it is now

  my ( $sec, $min, $hour ) = ( $now - $start, 0, 0 ); # seconds passed

  while ( $sec > 3600 ) { $sec -= 3600; $hour++ }     # hours
  while ( $sec > 60   ) { $sec -= 60  ; $min++ }      # minutes

  printf ("\r%02d:%02d:%02d",$hour,$min,$sec);        # print time
  sleep($gap);                                        # wait a while
}
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It probably positions the cursor using something like Curses or on of the Term::* modules if you are on a Unix-like system.

/J\

# UNIX style:
perl -e '$|++;for(1..10){ print $_;sleep 1; print "\b"}'

# dos style:
perl -e "$|++;for(1..10){ print $_;sleep 1; print \"\b\"}"
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Except of course that doesn't work for output of more than one character - try printing 10 - 25 for instance. I guess you could change your "\b" to "\r" if all is required is to return to the beginning of the line.

/J\

What does "\r" do on a mac?

This works fine for single digits, but if you increase the for loop to 15, the output gets all wonky:

perl -e '$|++;for(1..15){ print $_;sleep 1; print "\b"}
# prints:
1111115
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because, of course, you're only backspacing over one character. To avoid extra logic to figure out how many spaces to go back, you can just go to the beginning of the line with \r:
perl -e '$|++;for(1..15){ print $_;sleep 1; print "\r"}

Which one is "better" depends on your desired use, obviously.

Addendum: Ooog, slow typing today.

Basically, xorl, "\b" backs up one char, while "\r" rewinds the whole line :-)

Remember, if you print a newline character, you won't be able to use these control characters to erase it.

...
for(1..15){ print $_;sleep 1; print "\b"x(length$_)}
...
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