After much testing and log review I finally figured this out. I previously tried creating a non-blocking shell, and then writing to the shell, but was getting the same result as the above code. From looking through logs on CUCM I did notice that the script was creating tty=unknown shell. After setting the tty (->pty(1)), I was finally able to work everything out.

my $channel = $ssh2->channel();
$channel->blocking(0); 
$channel->pty(1);      #set tty
$channel->shell();     

sleep 10;   #required as the CUCM CLI is slow to start

$channel->write("show account\n");
sleep 5;    #Again the CLI is kind of slow

my $output;
$channel->read($output,128);
print "Account details:\n $output\n";
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