use Net::SSH::Perl; 
my $ssh;
$ssh = new Net::SSH::Perl("192.168.128.4"); $ssh->login("root"); 
($x,$y,$z) = $ssh->cmd('pwd');
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add:

Use warning;
Use strict;
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Also, you may want to change:

$ssh = new Net::SSH::Perl("192.168.128.4");
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To:

$ssh = Net::SSH::Perl-> new("192.168.128.4");
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And, is this your entire code? Are all your permissions and passwords the same on each box you're testing?
The third box may require a passwd in the line:

$ssh->login("root");
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following "root"