From looking at my code and yours it seems that you are missing an SPAGAIN; directive after the perl_call_sv() call.

Here's what I use in DBD::Sybase:

    if(imp_dbh->err_handler) {
        dSP;
        int retval, count;
        
        ENTER;
        SAVETMPS;
        PUSHMARK(sp);
        
        
        XPUSHs(sv_2mortal(newSViv(CS_NUMBER(errmsg->msgnumber))));
        XPUSHs(sv_2mortal(newSViv(CS_SEVERITY(errmsg->msgnumber))));
        XPUSHs(sv_2mortal(newSViv(0)));
        XPUSHs(sv_2mortal(newSViv(0)));
        XPUSHs(&sv_undef);
        XPUSHs(&sv_undef);
        XPUSHs(sv_2mortal(newSVpv(errmsg->msgstring, 0)));
        if(imp_dbh->sql)
        XPUSHs(sv_2mortal(newSVpv(imp_dbh->sql, 0)));
        else
        XPUSHs(&sv_undef);

        XPUSHs(sv_2mortal(newSVpv("client", 0)));
        
        PUTBACK;
        if((count = perl_call_sv(imp_dbh->err_handler, G_SCALAR | G_EV
+AL)) != 1)
        croak("An error handler can't return a LIST.");
        SPAGAIN;

        if(SvTRUE(ERRSV)) {
        POPs;
        retval = 1;
        } else {
        retval = POPi;
        }
        
        PUTBACK;
        FREETMPS;
        LEAVE;
        
        /* If the called sub returns 0 then ignore this error */
        if(retval == 0)
        return CS_SUCCEED;
    }
[download]

This works fine for me (and I have similar code in Sybase::DBlib and Sybase::CTlib.)

Michael