The synopsis in Net::SSH2 gives an example of what you want to do.

# Shamelessly cut and paste from said synopsis;)
use Net::SSH2;
my $ssh2 = Net::SSH2->new();

$ssh2->connect('example.com') or die;

if ($ssh2->auth_keyboard('fizban')) {
    my $chan = $ssh2->channel();
    $chan->exec('program');

    my $sftp = $ssh2->sftp();
    my $fh = $sftp->open('/etc/passwd') or die;
    print $_ while <$fh>;
}
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