Corion hit the nail on the head.

The brief answer is -- make your standard output (STDOUT) flush immediately. To do so, add this line near the beginning of your script (before doing any print statments):

    $| = 1;
[download]

The corresponding section from the article Corion linked to is the one entitled "Disabling Inappropriate Buffering", where it talks about making the "filehandle hot".

If you have use for a really fancy ascii-oriented progress bar, here is something I wrote that I've found useful in a lot of different applications:

#
#  progress_bar
#
#  Input:  $1 - A message to display with the progress bar.  It can co
+ntain
#               any of the following special 3-character strings, each
+ of which
#               will be converted to the following value:
#
#                   <b> - the progress bar itself
#                   <c> - the current count
#                   <p> - the percentage finished
#                   <r> - estimated remaining time
#                   <t> - the total count
#
#          $2 - A total count
#          $3 - (optional) The 'done' symbol (default is '@')
#          $4 - (optional) The 'todo' symbol (default is '-')
#
#  Output:  A closure that takes a count, and updates the progress bar
+ based
#           on its value.  When the progress bar is done, it should be
+ called
#           with no arguments, to show that the progress is complete -
+and- to
#           print a final newline.
##
sub progress_bar {
    my ($msg, $total, $done_sym, $todo_sym) = @_;

    ($total || 0) or fatal("total can NOT be zero/undefined!");
    $done_sym ||= '@';
    $todo_sym ||= '-';
    $| = 1;

    my $start  = time();
    my $last   = $start;
    my $remain = "???";
    my $b_left = ($msg =~ /<r>/)? 1: 0;

    my $c_prog = sub {
        my ($count) = @_;
        my $b_done = defined($count)? 0: 1;
        $b_done and $count = $total;
        ($count > $total) and $count = $total;
        my $pcnt    = sprintf "%6.2f", 100.0 * $count / $total;
        my $new_msg = $msg;

        # Calculate estimated remaining time
        my $time  = time();
        my $dtime = $time - $start;
        if ($b_done) {
            $remain = "Finished";
        } elsif ($b_left and $count and $dtime and $time - $last > 3) 
+{
            $last    = $time;
            my $rate = ($count / $dtime);
            my $nsec = int(($total - $count) / $rate);
            my $hr   = my $min = 0;
            if ($nsec > 3600) { $hr  = int($nsec / 3600); $nsec -= 360
+0 * $hr }
            if ($nsec > 60)   { $min = int($nsec / 60);   $nsec -= 60 
+* $min  }
            if ($hr) {
                $remain = sprintf "%02d:%02d:%02d", $hr, $min, $nsec;
            } elsif ($min) {
                $remain = sprintf "%02d:%02d", $min, $nsec;
            } else {
                my $s   = (1 == $nsec)? "": "s";
                $remain = "$nsec second$s";
            }
        }

        $new_msg =~ s/<c>/$count/g;
        $new_msg =~ s/<p>/$pcnt/g;
        $new_msg =~ s/<t>/$total/g;
        $new_msg =~ s/<r>/$remain/g;
        my $len   = 79 + 3 - length($new_msg);
        my $ndone = int($len * $count / $total);
        my $ntodo = $len - $ndone;
        my $bar   = ($done_sym x $ndone) . ($todo_sym x $ntodo);
        $new_msg =~ s/<b>/$bar/;
        print "$new_msg\r";
        $b_done and print "\n";
    };

    return $c_prog;
}
[download]

You can call it like this ... let's say you were processing a bunch of files, and wanted progress indication for each file processed:

use strict;
use warnings;

my @files    = ( 'some', 'list', 'of', 'files' );
my $nfiles   = @files;
my $nhandled = 0;
my $c_prog = progress_bar("File #<c> of <t> [<p>%] <b>", $nfiles);
foreach my $file (@files) {
    do_something_with_this_file($file);
    $c_prog->(++$nhandled);
}

$c_prog->();
[download]

Note that you don't want to print anything while the progress bar is displaying, or it will disrupt the progress bar's output. :)

Subroutine do_something_with_this_file left as an exercise to the reader.

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