I tried it with chan->blocking(0) disabled ...

I think that chan->blocking(0) is the correct thing to do. It works fine that way for me.
The while{} block at the end of your code puzzles me (and emits a couple of warnings). Is there a problem with coding it simply as:

use strict; 
use Net::SSH2; 

my $ssh2 = Net::SSH2->new(); 
$ssh2->connect('host')
 or die "Unable to connect host $@ \n"; 
$ssh2->auth_password('user','pass'); 
my $buflen = 512;
my $buf2 = '0' x $buflen;
my $chan = $ssh2->channel();
$chan->blocking(0);
$chan->shell();
$chan->write("ls\n");
select(undef,undef,undef,0.25);
my $len = $chan->read($buf2,$buflen);
print $buf2, "\n";
[download]

Update: Another option is to use exec() instead of shell():

my $chan = $ssh2->channel(); 
$chan->exec('ls'); 
$chan->read($buf2, $buflen);
print $buf2, "\n";
[download]

With this option, blocking is not disabled, and no select call needs to be made.

(Btw, I didn't know that the shell method could be made to work with a select() call .... so I've learnt something today.)

Cheers,
Rob