The "error" was in toke.c at S_scan_pat:

STATIC char *
S_scan_pat(pTHX_ char *start, I32 type)
{   
    PMOP *pm;
    char *s;

    s = scan_str(start,FALSE,FALSE);
    if (!s)
        Perl_croak(aTHX_ "Search pattern not terminated");

    pm = (PMOP*)newPMOP(type, 0);
    if (PL_multi_open == '?')
        pm->op_pmflags |= PMf_ONCE;
    if(type == OP_QR) {
        while (*s && strchr("iomsx", *s))
            pmflag(&pm->op_pmflags,*s++);
    }
    else {
        while (*s && strchr("iogcmsx", *s))
            pmflag(&pm->op_pmflags,*s++);
    }
    pm->op_pmpermflags = pm->op_pmflags;
    
    PL_lex_op = (OP*)pm;
    yylval.ival = OP_MATCH;
    return s;
}

Getting rid of the "o" fixed it.

japhy -- Perl and Regex Hacker