You should try

my $unamestring = $t->cmd("uname -a");
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instead. The example in the Net::Telnet documentation uses a command (who) that returns several lines, that's why it's using an array. But you don't need to, because you're running a command that returns only one line.
By the way, would you have used "use strict" and "use warnings" ( as everybody should get used to ) you'd certainly receive an appropriate warning.