see:
    PL_in_my = KEY_my

(op.c)
PADOFFSET
Perl_allocmy(pTHX_ const char *const name, const STRLEN len, const U32 flags)

  it calls:
    off = pad_add_name(name, len, ...)

(toke.c)

    pl_yylval.opval = newOP(OP_PADANY, 0);
    pl_yylval.opval->op_targ = allocmy(PL_tokenbuf, tokenbuf_len, 0);