It's not just qr.

Seems like Devel::Cover can cope only with constants as RHS of an or-condition, or the other way round, an op as the RHS screws things. Running your modified code

  $self->{ name  } = $args{ name  } || do { 'default'};
  $self->{ value } = $args{ value } || do { '' };
  $self->{ chars } = $args{ chars } || do { qr{^[a-z]+$} };
[download]

which passes fine, I get

---------------------------- ------ ------ ------ ------ ------ ------
+ ------
File                           stmt   bran   cond    sub    pod   time
+  total
---------------------------- ------ ------ ------ ------ ------ ------
+ ------
cover.pl                      100.0    n/a   66.7  100.0    n/a  100.0
+   95.3
Total                         100.0    n/a   66.7  100.0    n/a  100.0
+   95.3
---------------------------- ------ ------ ------ ------ ------ ------
+ ------
[download]

<update> From that output it seems that for e.g. $a || op($b) Devel::Cover erroneously adds an extra code path coverage branch for op to the conditional's to-be-covered count. You test both cases for the conditional, that makes 2 out of 3 "cases" = 66.7% coverage. </update>

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}