errmsg() returns the error of the Net::Telnet object itself, not that of the command/script you executed. For example:

use Net::Telnet;

use strict;
use warnings;

my $t = new Net::Telnet(timeout => 10, Errmode => "return", Prompt => 
+'/\[AAUA1\]/');
$t->open("ofgaix1");
$t->login("foo", "bar");
print "errmsg: " . $t->errmsg . "\n";
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Host ofgaix1 does not exsit, and when I tested it, it printed this:

errmsg: login failed: filehandle isn't open
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The error message if they get printed on the server side, then they are returned from cmd() call in an array, together with all other print outs.