use strict;
use warnings;
require IO::Socket::SSL;
require Mail::IMAPClient;

# Gmail does not provide a non-SSL IMAP server.
my $socket = IO::Socket::SSL->new("imap.gmail.com:993") 
my $imap = Mail::IMAPClient->new(  
    Server  => 'imap.gmail.com'
    User    => 'you@gmail_labs.com',
    Password=> $pass,
    Socket  => $socket,
) or die "Cannot connect to $host as $id: $@";

my %address_list;
my @messages = $imap->messages();

foreach my $message (@messages) {
    my $envelope = $imap->get_envelope($message);
    my $from = $envelope->from();
    foreach my $from_address (@{$from}) {
        $address_list{$from_address} = 1;
    }
    my $to = $envelope->to();
    foreach my $to_address (@{$to}) {
        $address_list{$to_address} = 1;
    }
    my $cc = $envelope->cc();
    foreach my $cc_address (@{$cc}) {
        $address_list{$cc_address} = 1;
    }
    my $replyto = $envelope->replyto();
    foreach my $replyto_address (@{$replyto}) {
        $address_list{$replyto_address} = 1;
    }
    my $sender = $envelope->sender();
    foreach my $sender_address (@{$sender}) {
        $address_list{$cc_address} = 1;
    }
}

$imap->logout();

my @address_list = keys %address_list;
...
[download]

Note that this will only catch message in the root IMAP folder ATM - although it can be easily extended. But then, does GMail have real "folders" as such?