use Net::SFTP::Foreign;
use File::Temp;

my $hostname = 'localhost';

my $ssherr = File::Temp->new or die "File::Temp->new failed";

open my $stderr_save, '>&STDERR' or die "unable to dup STDERR";
open STDERR, '>&'.fileno($ssherr);

my $sftp = Net::SFTP::Foreign->new($hostname, more => ['-v']);

# the child ssh process has already been created with the redirected STDERR
# so we can reset it on the parent:
open STDERR, '>&'.fileno($stderr_save);

if ($sftp->error) {
  print "sftp error: ".$sftp->error."\n";
  seek($ssherr, 0, 0);
  while (<$ssherr>) {
    print "captured stderr: $_";
  }
}

close $ssherr;