I also tend to use maketext, as there are utilities like poEdit to handle the language files.

As for your other questions:

• localizing $_ relates to scoping problems of the variable $_. It doesn't have to do anything with localization.
• I haven't seen a non-English error message from Perl so far. This is not a huge deal, as users shouldn't see Perl's messages anyway.
• if you use locale, a number of things will be adapted to the current locale. For example, in a German locale, sprintf() will format a floating point number as 1,25 instead of 1.25. strftime (from the POSIX module) will format date and time strings correctly according to the current locale (e.g. Mi 13. Januar 2004, 17:52 instead of Wed, January 13th 2004, 5:52pm. Also, the sorting order will be affected by the locale (e.g. in sort).

Go and read perllocale, it has some more interesting information. For example, you can also find out the currency symbol of the current locale so you can print ¥100 or €100 instead of $100.

One last hint: Whatever approach you use, use a format that comes with tools like poEdit or is easy to understand. The better your software is and the easier it is to translate it, the more people will volunteer to translate it to another language (talking in a open source context, not business, of course).