At one point, I was just passing the raw file descriptor into the perl callback as an integer, like so:

int conn = socket(.....);

[...]

ENTER;
SAVETMPS;
PUSHMARK(SP);
XPUSHs(sv_2mortal(newSViv(conn)));
PUTBACK;
cpretval = call_sv(the_coderef, G_SCALAR|G_EVAL|G_KEEPERR);
SPAGAIN;
if(SvTRUE(ERRSV)) {
    STRLEN n_a;
    log_helper(1,kidno,SvPV(ERRSV, n_a));
    POPs;
    real_retval = 0;
}
else {
    if(cpretval != 1)
        croak("Return count from coderef callback != 1");

    real_retval = POPi;
}
PUTBACK;
FREETMPS;
LEAVE;
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And then in the called-back perl code being called, I did:

sub the_callback_function {
  my ($conn) = @_;

  open(my $c,"+<&=$conn");
  bless $c, 'IO::Socket::INET';
  $c->autoflush(1);
  $c->send("Network data!\n");
  .....
[download]

And surprisingly that worked fine, and sent "Network data!\n" out over the open socket. I'm sure there are some grave issues lurking in my heathen blessing of the filehandle to IO::Socket::INET without using any of its constructor code if I started using some of the other methods, but that's not terribly important at the moment.

So my next step was to eliminate the neccesity of having the perl side of things use that funky open(my $c,"+<&=$conn"); line by having the C code send an actual perl filehandle instead of a raw integer. I've gotten halfway there (I think), by creating a PerlIO* out of my integer in C, using:

PerlIO* conn_pio;
conn_pio = PerlIO_fdopen(conn,"r+");
[download]

However, attempting to do things like XPUSHs((SV*)conn_pio) are of course failing to do jack. How exactly does one properly push a PerlIO object onto the argument stack? It seems like there should be a function like io_2sv() to match the one I found named sv_2io(), but I don't see it, or anything that looks like it. Any ideas?