From the discussion in your related thread it is not clear at all that you really are facing a problem involving ptys. Maybe you should make sure first that this is the case?

Otherwise, this follow-up exposes an XY Problem. You were given some hints, and questions whose answers will help us to help you track down the problem. What about those?

     # I'm stuck here. Don't know how 
     # to construct the command correctly...
     run \@cmd, '<pty<', \$in,  '>pty>', \@output ;
[download]

According to the documentation of IPC::Run

     # Redirecting using psuedo-terminals instad of pipes.
     run \@cat, '<pty<', \$in,  '>pty>', \$out_and_err ;
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--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}