Try this:

#!/usr/bin/perl -w

our $MAILBOX = './mail/addresses';

use strict;
use Email::Find;
use IO::File;

# Read the mailbox
my $fh = IO::File->new($MAILBOX)
  or die "unable to read mailbox '$MAILBOX': $!";
my $mail;
{
  local $/;
  $mail = <$fh>;
}

# Find addresses
my %addy;
my $finder = Email::Find->new(
  sub {
    my($email, $orig_email) = @_;

    $addy{ $email->address }++;
    return $orig_email;
  }
);
$finder->find(\$mail);

foreach my $address (keys %addy) {
  print "$address $addy{ $address }\n";
}
[download]

You just needed to read the mailbox file so you could pass it to the find() method.

You should also become familiar with the Mail::Address module, since that's what is passed to your callback routine.

You didn't indicate what you actually wanted to do with the addresses, so in this example I'm just storing them in a hash.