I would say it "questionable" as well. (And I use it all the time. :-) Three reasons.
1. I try to avoid global variables as much as possible.
2. $" is line noise and accordingly easy to miss by a maintainer (who is most likely not at my level of perl).
3. Beginners are much more likely to set global variables than to localize them.
(4. $" screws up the syntax highlighting in many editors.)

Join() performs the same task in a much more maintainable way, so its not really needed. Also, if you think about it join is about the same or less in terms of usage.

local $"=','; my $s="@list";
my $s=join(',',@list);
[download]

As i said though I use it in obfus, snippets and temporary debuging code all the time. But only cause I like to. :-)

Yves
--
You are not ready to use symrefs unless you already know why they are bad. -- tadmc (CLPM)