Fix "useless interpolation" warnings in the code base

... and (where spotted) fixed a couple of error messages.

Signed-off-by: Niels Thykier <niels@thykier.net>
t: Enable perl critic test by default (with reduced policy)

Instead of having a strict policy we never enforce, have a small
policy we currently follow and can enforce.

Signed-off-by: Niels Thykier <niels@thykier.net>