WORKING:
...
debug1: Sending command: /usr/bin/ksh -c "cat /etc/passwd"
debug2: channel 0: request exec confirm 1
debug2: fd 4 setting TCP_NODELAY
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_confirm: type 99 id 0
debug2: exec request accepted on channel 0
debug2: channel 0: rcvd eof
debug2: channel 0: output open -> drain
debug2: channel 0: obuf empty
debug2: channel 0: close_write
debug2: channel 0: output drain -> closed
...

NOT WORKING:
...
debug1: Sending command: /usr/bin/ksh -c "cat /etc/passwd"
debug1: channel request 0: exec
debug1: fd 4 setting TCP_NODELAY
debug2: callback done
debug1: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel 0: rcvd adjust 131072
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: channel 0: write failed
debug1: channel 0: close_write
debug1: channel 0: output open -> closed
debug1: channel 0: rcvd eof
debug1: channel 0: rcvd close
debug1: channel 0: close_read
debug1: channel 0: input open -> closed
debug3: channel 0: will not send data after close
debug1: channel 0: almost dead
debug1: channel 0: gc: notify user
debug1: channel 0: gc: user detached
debug1: channel 0: send close
debug1: channel 0: is dead
debug1: channel 0: garbage collecting
debug1: channel_free: channel 0: client-session, nchannels 1
debug3: channel_free: status: The following connections are open:
  #0 client-session (t4 r0 i3/0 o3/0 fd -1/-1)
debug3: channel_close_fds: channel 0: r -1 w -1 e 7
debug1: fd 1 clearing O_NONBLOCK
debug1: fd 2 clearing O_NONBLOCK
Connection to host closed.
debug1: Transferred: stdin 0, stdout 0, stderr 30 bytes in 15.1 seconds
debug1: Bytes per second: stdin 0.0, stdout 0.0, stderr 2.0
debug1: Exit status 0
...

##</code><code>##

use strict;
use warnings;

use IO::Select;

my $cmd = '"cat /etc/passwd"';
my $login = 'usr@host';
my $pkey = "$ENV{HOME}/.ssh/privateKey";

my @shell = ( '/usr/bin/ksh' => (
	-c => $cmd,
));

my @sshcmd = ('/usr/bin/ssh' => (
	-t => (),
	-v => (),
	#-vv => (),
	#-vvv => (),
	-i => $pkey,
	-o => 'StrictHostKeyChecking no',
	$login,
	@shell,
));

open (IFILE, '-|') || exec @sshcmd;

my $select = IO::Select->new();
$select->add(\*IFILE);

my @ready;
while ( @ready = $select->can_read(5) ) {
	foreach my $fh (@ready) {
		my $line = <$fh>;
print STDERR "line=$line";
		if(eof($fh)) {
			$select->remove($fh);
			close($fh) || warn "Close problem on fh: $fh";
		}
	}
}

close(IFILE);

END{
	print STDERR "\$!: $!\n";
}