stored in your SV. The "p" stands for private.
There are various ways in which the private and public flags may differ.
-For example, a tied SV may have a valid underlying value in the IV slot
-(so SvIOKp is true), but the data should be accessed via the FETCH
-routine rather than directly, so SvIOK is false. Another is when
+For example, in perl 5.16 and earlier a tied SV may have a valid
+underlying value in the IV slot (so SvIOKp is true), but the data
+should be accessed via the FETCH routine rather than directly,
+so SvIOK is false. (In perl 5.18 onwards, tied scalars use
+the flags the same way as untied scalars.) Another is when
numeric conversion has occurred and precision has been lost: only the
private flag is set on 'lossy' values. So when an NV is converted to an
IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.