You could wait for the next prompt after having issued the listdev command, and then print out the accumulated buffer contents, accessible via before(). Something like

...
print $ssh "listdev\n";

unless ($ssh->expect(10,"sysadmin>")) {
        die "listdev command never finished, " .$ssh->exp_error()."\n"
+;
        }

my $out = $ssh->before();
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