This is an ideal application for threading if that option is available to you.

#! perl -sw
use strict;
require 5.008;
use threads 'yield';
use threads::shared;

use vars qw[$ITERS];
$ITERS ||= 10000;

$|=1;

my $percent : shared = 0;

sub progress{
    print '[', ' ' x 80, ']', $percent;
    while( $percent < 1 ) {
        printf "\r[%s%s] %5.2f%% ", '#' x (80*$percent), ' 'x(80*(1-$p
+ercent)), 100*$percent;
        yield;
    }
    print "\n";
    return;
}

sub TakesALongTime{
    my $in    =  0;
    my $max = shift || 10000;

    for my $count ( 1 .. $max ) {
        $percent = $count/$max;
        rand () ** 2 + rand () ** 2 < 1 && $in ++ ;
        yield;
    }
    return sprintf '%f', 4 * $in / $max;
}

my $thread = threads->create('progress');

my $PI_approx = TakesALongTime($ITERS);
$thread->join;

print "After $ITERS iterations, pi was approximated to be: $PI_approx\
+n";

__END__
D:\Perl\test>261528 -ITERS=100000
[#####################################################################
+##########] 100.00%
After 100000 iterations, pi was approximated to be: 3.136440
[download]

Not a great demo, but if any of it needs explaination, please ask. (See Monte Carlo approximation of PI for Abigail's Monte Carlo approximation of PI snippet.)