IO *io = sv_2io( sv );
PerlIO *perlio = IoIFP( io );
if( perlio != NULL ) {
    PerlIO_read( perlio, ... );
}
else {
    const MAGIC *mg = SvTIED_mg( (SV *) io, 'q' );
    if( mg != NULL ) {
        sv = SvTIED_obj( (SV *) io, mg );
        // call the READ method with sv as object...
    }
    else {
        Perl_croak( aTHX_ "invalid IO handle" );
    }
}