Not a complete explanation... but my suspicion is that it has to do with Net::SSH::Expect globally setting $SIG{CHLD}. At least I do get the same error when replacing the use Net::SSH::Expect; in your code with

BEGIN {
    package Net::SSH::Expect;
    use POSIX qw(:signal_h WNOHANG);

    $SIG{CHLD} = \&reapChild;
    
    sub reapChild {
       do {} while waitpid(-1,WNOHANG) > 0;
    }
}
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(which is extracted as is from Net::SSH::Expect)

When I localize the handler

    local $SIG{CHLD} = \&reapChild;
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things do work for me... (i.e. there's no interaction any longer).