So... I promised a bit too much.  It is not possible to override an
individual indentation setting (e.g. cperl-indent-level) when a
cperl-file-style is used.  File (and directory) variables are applied
after the body of cperl-mode, and the styles do their work only after
that in hack-local-variables-hook.  I did not know that.  So, styles
will always override the settings they provide.

But apart from that, the patch (attached) does what I would expect:  I
can set a default style in custom, individual styles from .dir-locals.el
or file local variables work without affecting other buffers.

I suggest this patch should go to the emacs-29 branch.

--
Cheers,
haj