if I send a command to a ftp process, and I want to store it's (sic) return value

Are you just doing FTP? Do you need to use Expect, or is there some reason why you can't/don't want to use Net::FTP? The code will be much simpler:

my $ftp = Net::FTP->new('foo.example.com')
  or die "can't connect: $@";

$ftp->login( 'leet', 'seKreT' )
  or die 'can't login: ' . $ftp->message;

$ftp->cwd("/pub/foo/bar")
  or die 'cannot set cwd' . $ftp->message;

my @files = $ftp->dir();
$ftp->quit;
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