Can't call method "error" on an undefined value at /usr/local/share/perl/5.26.1/Net/SFTP/Foreign/Backend/Net_SSH2.pm line 43.
$ cat 5.ssh2_1.pl 
...
my $block1 = 1;
my $block2 = 1;
my $block3 = 0;

my $chan = $ssh2->channel();
$chan->blocking($block1);
$chan->exec('ls -la');
while (<$chan>) { print }
$chan->close;

#shell use
my $chan2 = $ssh2->channel();
$chan2->blocking($block2);
$chan2->shell();
print $chan2 "uname -a\n";
print "LINE : $_" while <$chan2>;
$chan2->close;

say "execution goes through here";
my $chan3 = $ssh2->channel();
$chan3->blocking($block3); ##line 43
$chan3->exec('pwd');
while (<$chan>) { print }
$chan3->close;