I don't think that'll help, as the OP said they want to stream the output of the command, while backticks wait until the external command has completed.

Sure

Just a guess, try my $cmd = 'nohup /usr/sbin/sshuttle ...' although that may prevent you from capturing its output, not sure if that's what you want or not.

Unfortunately in real program $cmd comes as users input parameter which I can't alter or modify

I suspect there's something going on in the IPC interaction between sshuttle and perl calling it

Yeah, this what I tried to say at the very beginning. There are some rare cases ( rare external commands to interact with ) where Perl/IPC/open ( $cmd |) "hangs" ...