(gettext.info)Special Keywords

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15.5.18.3 How to Extract Hash Keys
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   Translating messages at runtime is normally performed by looking up
the original string in the translation database and returning the
translated version.  The “natural” Perl implementation is a hash lookup,
and, of course, ‘xgettext’ supports such practice.

     print __"Hello world!";
     print $__{"Hello world!"};
     print $__->{"Hello world!"};
     print $$__{"Hello world!"};

   The above four lines all do the same thing.  The Perl module
‘Locale::TextDomain’ exports by default a hash ‘%__’ that is tied to the
function ‘__()’.  It also exports a reference ‘$__’ to ‘%__’.

   If an argument to the ‘xgettext’ option ‘--keyword’, resp.  ‘-k’
starts with a percent sign, the rest of the keyword is interpreted as
the name of a hash.  If it starts with a dollar sign, the rest of the
keyword is interpreted as a reference to a hash.

   Note that you can omit the quotation marks (single or double) around
the hash key (almost) whenever Perl itself allows it:

     print $gettext{Error};

   The exact rule is: You can omit the surrounding quotes, when the hash
key is a valid C (!)  identifier, i.e. when it starts with an underscore
or an ASCII letter and is followed by an arbitrary number of
underscores, ASCII letters or digits.  Other Unicode characters are
_not_ allowed, regardless of the ‘use utf8’ pragma.