While i'm not familiar with MCE::Hobo (or Inline::Java for that matter), it seems it uses fork() to create new workers. While this works great in general, there are a few caveats that you have to keep in mind and check for.

For once, open file handles are shared with child processes. That means you might need to close all file handles and open new ones. Including any temporary files Inline::Java might have created. For example, Inline::Java might create some external temporary files that it can execute with the java engine in the parent process during loading of your module. When the first child process exits, it might clean up (delete) those files, thus preventing other child processes from working correctly. And/or it might share a network socket to the JVM.

Not sure how to solve this in Inline::Java, or even if it's solvable in it's current form. There is a thread on stackoverflow discussing a similar problem.

One way to work around this would be to have a master process that forks and then calls up a completely new instance of the worker script. Something along the lines of (untested, written from memory)

#!/usr/bin/env perl
use strict;
use warnings;
use Time::HiRes qw(sleep);

my $workercount = 0; # Counts active workers

SIG{CHLD} = sub {
    # Child exit detected
    $workercount--; # remove child from "active" count
    return;
}

while(1) {
    if($workercount < 20) {
        my $pid = fork();
        if($pid) {
            # parent
            $workercount++; # forked a new child
        } else {
            # child
            my $cmd = 'perl myworkerscript.pl';
            eval {
                exec($cmd);
            };
            exit(0);
        }
    } else {
        sleep(0.5);
    }
}
[download]

There is also the funny matter of things like the pseudo-random number generator. When you fork(), all child processes will generate the same sequence of random numbers. Some protocols (and other stuff) use randomly generated unique IDs to, say for example, identify a transaction or a data block. If you launch multiple child processes that now generate the same "unique" IDs, this could lead to all sorts of strange problems. Calling srand() immediately after fork in the child (in whatever function you call from MCE::Hobo) might be a good idea.

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