I'm not sure this can be done on a system that doesn't have IPC::Open2 without knowing a lot more about the OS. It is precisely why IPC::Open2 was written afaik. For example, in linux you could use pipes to kinda fool it.

Probably the most straight forward solution would be to open my $outputter, "|program > tmpfile", and then open the tmpfile when you're done. You know, local $/ = undef; my $entire_file = <$inputter>.