Just for the record, here's the final code from the actual script it's being used in, which works correctly. This subroutine forks off an external command and returns a filehandle that you can read and write to, and convinces the external program that it's running in a terminal.

use IO::Handle;
use IO::Pty;
use POSIX;

sub forkptycmd($) {
  my $cmd = shift;
  my $pty = new IO::Pty;
  my $pid = fork;
  if(!defined($pid)) { die "error forking: $!"; }
  if($pid==0) {
    POSIX::setsid();
    my $slave  = $pty->slave;
    close($pty);
    STDOUT->fdopen($slave,'>') || die $!;
    STDIN->fdopen($slave,'<') || die $!;
    STDERR->fdopen(\*STDOUT,'>') || die $!;
    close($slave);
    exec($cmd);
  }
  $pty->close_slave();
  return $pty;
}
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