After a talk with Nei on #gtk-perl I’ve come to a few conclusions. For one, I’m really bad at this whole locale thing. For two, this whole locale thing is pretty much broken by design.

There are a few things working together that make this so bad:

• GTK will call setlocale(LC_ALL, "") when it starts, so we were mistaken about the use of the Perl instruction POSIX::setlocale — it should, by all means, go after the Gtk init, so as to actually override whatever Gtk loaded from the environment
• C’s locale support is pretty much broken: there’s apparently no way to say “this is something user-facing, please present it as appropriate” and “this is something that must remain exactly the way I’m saying it”.
• Perl will inherit this behaviour and unless the libraries dealing with numbers setlocale(LC_NUMERIC, "C"), they will end up producing localised numbers the instance they turn it into a string. Which many do.
• JSON gets slightly confused, it appears, by producing a JSON string like { cmd: "something", ts: 1373556417,044533, data: { ... } }

So, for me, the solution is turning locales off on numerals. For others, it would require calling setlocale back and forth. Here is another example of this issue cropping up all of a sudden.