You're right,
Net::SSH::Perl is not to be used with Expect, but you can use Net::SSH though. Net::SSH::Perl has built-in support for the authentication protocols, so there's no longer any hassle of communicating with any external processes.

from the docs:

my $ssh = Net::SSH::Perl->new("host1");
    $ssh->login("user1", "pass1");

    $ssh->cmd("foo"); #I'd say: try su?
[download]

Of course I could be oversimplifying things, I haven't tested it.

Teabag