Update: I've managed to get it working - the C code now passes in a real filehandle glob which is already blessed to IO::Socket::INET. However, I'm not entirely sure that my code to do so is entirely sane and correct. It's more or less what I managed to cobble together by reading the source to pp_socket in pp_sys.c and staring at the perlapi/perlguts/etc docs. I don't entirely understand the purpose of some things, and I'm not sure that I'm not leaking memory:

[...]

IO* conn_pio;
SV* connrv;
GV* fhglob;
HV* our_stash;
HV* isn_stash;

isn_stash = gv_stashpv("IO::Socket::INET",1);
our_stash = gv_stashpv("MyModule::Name",1);
fhglob = gv_fetchmethod(our_stash,"myfilehandle");
conn_pio = GvIOn(fhglob);
IoIFP(conn_pio) = PerlIO_fdopen(conn, "r"SOCKET_OPEN_MODE);
IoOFP(conn_pio) = PerlIO_fdopen(conn, "w"SOCKET_OPEN_MODE);
IoTYPE(conn_pio) = IoTYPE_SOCKET;
connrv = newRV_inc((SV*)fhglob);
sv_bless(connrv,isn_stash);

ENTER;
SAVETMPS;
PUSHMARK(SP);
XPUSHs(connrv);
PUTBACK;
cpretval = call_sv(kid_conn_coderef, G_SCALAR|G_EVAL|G_KEEPERR);

[....]
[download]