The trick is to keep track of the PID as you launch the process. On non-Windows systems, this is easiest done through the magic of exec, which replaces the current process by the other process, keeping the PID ($$) the same. So you can store $$ and then exec the "real" process:

sub launch {
    my ($pid_file,@process_and_args) = @_;
    open my $fh, '>', $pid_file # or print to STDOUT for easier handli
+ng by ssh?
       or die "Couldn't create '$pid_file': $!";
    print {$fh} $$;
    exec(@process_and_args)
        or die "Couldn't launch [@process_and_args]: $!";
};
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You have to decide whether to output the PID to a file or to directly output it to STDOUT and read it from your ssh launcher and store it in the controlling master.