On my Linux box, calling exec in a child process doesn't create a new process, at least pid-wise. That is, if you can replace the system call with exec, you can use the return value of the fork in the parent for your pid.

#! perl

use strict;
use warnings;

my $child_pid = fork();

die "Couldn't fork: $!\n" unless defined $child_pid;

if ($child_pid)
{
    print "Child pid is $child_pid\n";
}
else
{
    exec( 'java ...' );
}
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