B::COW provides some naive additional B helpers to check the COW
status of one SvPV.  COW or Copy On Write introduction

A COWed SvPV is sharing its string (the PV) with other SvPVs. It's a
(kind of) Read Only C string, that would be Copied On Write (COW).

More than one SV can share the same PV, but when one PV need to alter
it, it would perform a copy of it, decrease the COWREFCNT counter.

One SV can then drop the COW flag when it's the only one holding a
pointer to the PV.

The COWREFCNT is stored at the end of the PV, after the the "\0".

That value is limited to 255, when we reach 255, a new PV would be
created,