the send call doesn't return anything or even dies in a way I've been able to catch

Hm, it's hard to image that your script just disappears without a trace. Actually, I'd expect qmail to somehow check and report the status of invoking qmail-queue. Also, when you look in the sources of Mail/Qmail/Queue/Send.pm, there's plenty of error handling that would cause the program to die/carp with some helpful messages. But the question probably is "where do they end up?" :)

Have you checked the syslog and any qmail specific logfiles? (not exactly sure where they're kept (probably /var/log/qmail/* or /var/log/qmail.log) -- I'm not using qmail myself, and as qmail is in "non-free", there's no readily downloadable .deb package to check...)

If you absolutely can't find where the error messages are ending up, it might be easier to just dump them another time to a place where you'll actually look, e.g. somewhere in /tmp/ . In Mail/Qmail/Queue/Error.pm there's a routine _fail which seems to be called eventually for every error. So, you could insert a bit of debugging code in there

sub _fail(@)
{
    ### DEBUG
    if (open my $log, ">>", "/tmp/qmail-queue.log") {
        print $log "@_\n";  # or something like this
        close $log;
    } else {
        # writing another error message to nowhere if
        # this fails would be mostly useless...
    }
    ###

    my $default_ec = shift;
    ...
}
[download]

Otherwise, if this also isn't going to lead you to enlightenment, you could try strace(1) as a last resort. Either write a wrapper for your script which runs it under strace (this is the easy way, but it would likely produce lots of unnecessary output), or fork/exec strace in the background from within your script (attaching it to the PID of the script), immediately before you call $msg->send(...). (If you want help with the latter approach, just report back...)

Good luck!