...but you'd better make sure that you're actually writing to a terminal:

#!/usr/bin/perl

use strict;

my $in = shift || '/usr/share/dict/words';
my $running_on_terminal = -t STDOUT;
my $last_word_length;

open IN, $in or die "Cannot open $in for input: $!\n";

while( <IN> ) {
    chomp;
    if ($running_on_terminal) {
         my $length_diff = $last_word_length - length($_);
        $length_diff >= 0
            or $length_diff = 0;
        print ("\rProcessing $_..." . (" " x $length_diff)
               . ("\b" x $length_diff));
        $last_word_length = length($_);
    }

    # do something with $_
}
print "\n" if ($running_on_terminal);

close IN;
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caveat: this code still assumes the user's terminal is wide enough for any line we're going to print.

You might want to wrap it up into a module or object, so you can go:

my $pm = new ProgressMeter(-items => scalar @list);

$pm->start;
my $c;
for my $item (@list) {
    $pm->update(++$c);
}
$pm->finish;
[download]

I wrote something like that once, but unfortunately it is owned by the company I wrote it for :-(