because it exec doesn't return to the program, unless theres an error

That's because exec doesn't create a new process. It replaces the calling program with the specified program, running it in the same process.

In Linux/Unix/POSIX based systems, the PID returned by fork to the parent is the child you wait for (or kill, if need be).

 my $pid = fork();
 unless defined $pid {
    warn "Can't fork: $!\n";
    ...; # do something else
    exit;
 }
 if ($pid == 0) {
    # child
    exec @cmd_and_parameters;
    die "Could not exec $cmd_and_parameters[0]: $!\n";
 } else {
    # parent
    ...; # do other things
    waitpid($pid,0) == $pid or die "Error waiting for child: $!\n";
    ...; # do more (if needed)
 }
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Using, for example, Parallel::ForkManager will make this easier, as well as hide any ugly details for OSs that do not have fork (like MS Windows).