Yeah, the docs aren't that vague for Net::IMAP::Simple in my opinion. It says that $imap->list($number) returns the number of bytes. It'll also return all the sizes as an array @a = $imap->list if you call it without an argument.

Normally I wouldn't write out the whole thing (as I think it spoils most of the fun), but in this case I actually wanted it for my own purposes and it's really just a cut and paste from the docs anyway. (spoilers within)

use Mail::IMAPClient;

my @folders = qw( INBOX saved-messages folder1 folder2 );

my $imap = Mail::IMAPClient->new( Server => 'mail.server.com:143',
 User => 'user',
 Password => 'pass')
  or die "IMAP failure: $@";

foreach my $folder (@folders) {
  $imap->select($folder)
    or die "Select failure: $!";
  my @msgs = $imap->search('ALL')
    or die "Search error: $!";
  foreach my $msg (@msgs) {
    if ($imap->size($msg) > 100000) { # 100k
      $imap->delete_message($msg);
      next;
    }
  }
  $imap->expunge($folder);
  $imap->close($folder);
}
$imap->logout();
[download]

sub listCallback {
  my ($imap, $list) = @_;
  print($list->mailbox(), "\n");
}

my %options = ();
my $imap = new Net::IMAP($host, %options);
$imap->login($user, $pass);
$imap->set_untagged_callback('list', \&listCallback);
my $response = $imap->list('', 'INBOX.*');
# function listCallback prints mailbox names
[download]