FLAGS = (PADMY,IOK,pIOK)
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  FLAGS = (PADMY,IOK,POK,pIOK,pPOK)
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Because in the first Dump() call, $a didn't have a string in it. No POK flag. A scalar with all its data/content flags off (PADMY has nothing to do with data in the scalar), is undefined and the IV/NV/PV values, if allocated, are allocated but uninitialized memory at that point. The SV type, here PVIV, indicates what is allocated, but not what is currently in use. An undefined SV can be a SVt_PVMG or SVt_PVLV (largest scalar possible, after LV, the SV is not a Perl language Scalar anymore and any attempts to use a SV bigger than SVt_PVLV as a Perl Scalar will result in "bizarre copy of" panics), yet have no magic or Lvalueness attached. An undefined SV can be SVt_NULL, which has almost nothing allocated except the SV struct. I hope youve read atleast the first 1/3rd of illguts. Devel::Peek will show allocated but uninitialized data sometimes as a diagnostic tool so you can figure out if there is a mistake in the flags and the data is actually valid and you forgot a SvIOK_only when creating a IV SV the long way (sv_setiv is the short way). I'm not sure if the advanced fields of magic and LVs are shown if the SV isn't flagged as magic or LV by Devel::Peek.