First, you may have better luck using Net::SSH2. However, if for some reason you must use SSH1, it appears that you are using a passwordless login (implying that you are using trusted keys to login), but you said that you can not set up trusted hosts. Try using the example segment below from the Net::SSH::Expect docs for logging in with a password (includes error checking).

#!/usr/bin/perl

use strict;
use warnings;
use Net::SSH::Expect;

        #
        # You can do SSH authentication with user-password or without 
+it.
        #

        # Making an ssh connection with user-password authentication
        # 1) construct the object
        my $ssh = Net::SSH::Expect->new (
            host => "myserver.com", 
            password=> 'pass87word', 
            user => 'bnegrao', 
            raw_pty => 1
        );

        # 2) logon to the SSH server using those credentials.
        # test the login output to make sure we had success
        my $login_output = $ssh->login();
        if ($login_output !~ /Welcome/) {
            die "Login has failed. Login output was $login_output";
        }
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