I've researched this a little bit, and I'm not sure that we can work around it. Two things seem to be keeping us from properly inheriting from Bit::Vector:

1. It doesn't look like you can bless a scalar constant (one marked internally as read-only):

   perl -e '$a = \"constant"; bless($a, "Test");'
   Modification of a read-only value attempted at -e line 1.
   [download]

   This is probably normal behavior of Perl (even though I don't understand why), but the problem is that it looks as though Bit::Vector is claiming that its referent is read-only, presumably to prevent you from accidentally modifying its value:

   perl -MBit::Vector -e '$a = new Bit::Vector(8); ${$a} = 2;'
   Modification of a read-only value attempted at -e line 1.
   [download]

   This keeps us from re-blessing it into our own class.

2. The other problem is that Bit::Vector's "new" function is ignoring the first argument to "new", which means it doesn't allow itself to be called in an inherited style. Here's the code in Vector.xs that actually does the blessing:

   reference = sv_bless(sv_2mortal(newRV(handle)),  BitVector_Stash);
   [download]

   This forces the new object to be created in Bit::Vector, ignoring the class passed to the function (it's there in the code, just being ignored).

   I'm no XS expert by any stretch, but this change to Bit::Vector *might* work, which effectively does a stash lookup based on the first argument already passed into the function:

   reference = sv_bless(sv_2mortal(newRV(handle)), gv_stashpv(SvPV_nolen(
   +class), 1));
   [download]

   It makes some bad assumptions, though. I'd rather leave this up to the experts to fix properly, if a fix is even desired.

   (update: the above change breaks some of the tests for Bit::Vector, notably the use of $new = $old->new($bits) syntax to create a new Bit::Vector, but your test script does run as you expect.)

The author of the module is Steffen Beyer <mailto:sb@engelschall.com>. I might shoot him an e-mail to ask him if these behaviors and side-effects are intentional.