my $rg = $b_frame->RadiobuttonGroup(-list => [qw( stuff)])->pack();
[download]

$rg->{'SubWidget'}->{'stuff'}->configure(-background => 'pink');
[download]

--shmem.

Update:

The manual page states:

DESCRIPTION
   Displays a set of related radiobuttones with a frame in vertical or
+ hori-
   zontal orientation.

   All radiobuttones are advertised with the names given in the -list
   option.

   Any additional options which are given to this widget are applied t
+o all
   of the radiobuttons it manages.
[download]

This is not the case, so I call that a Bug or at least an inconsistency between object behaviour and manual page. You should report that to the author.

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