{
   package SFTP;

   use IO::Handle qw( );

   sub new {
      my $class = shift(@_);

      my ($to_sftp, $fr_sftp);
      # open2 dies on error.
      my $pid = open2($to_sftp, $fr_sftp, 'sftp', @_);

      $to_sftp->autoflush(1);

      return bless({
         pid      => $pid,
         to_sftp => $to_sftp,
         fr_sftp => $fr_sftp,
      }, $class);
   }

   sub DESTROY {
      my ($self) = @_;

      ...send exit command to child...
      ...kill child if necessary...

      waitpid($self->{pid}, 0);
   }

   sub do {
      my ($self) = @_;
      my $fr_sftp = $self->{fr_sftp};
      my $to_sftp = $self->{to_sftp};

      print $to_sftp "command";
      my $response;
      for (;;) {
         my $line = <$fr_sftp>;
         last if $line =~ $prompt;
         $response .= $line;
      }
      return $response;
   }
}

my $sftp = SFTP->new(...sftp args...);
my $response = $sftp->do("...\n");