Redirecting STDERR to a file is now natively supported by Net::SFTP::Foreign (version 1.58_01 required):

my $ssherr = File::Temp->new or die "File::Temp->new failed";
my $sftp = Net::SFTP::Foreign->new($hostname, more => ['-v'], stderr_f
+h => $ssherr);
if ($sftp->error) {
  print "sftp error: ".$sftp->error."\n";
  seek($ssherr, 0, 0);
  while (<$ssherr>) {
    print "captured stderr: $_";
  }
}
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