[1] GNU gettext utilities ( 版) <http://www.gnu.org/software/gettext/manual/gettext.html#perl_002dformat>
Perl brace format strings are described in the Locale::TextDomain(3pm) manual page of the CPAN package libintl-perl. In brief, Perl format uses placeholders put between braces (‘{’ and ‘}’). The placeholder must have the syntax of simple identifiers.
[2] Locale::TextDomain - search.cpan.org ( 版) <http://search.cpan.org/dist/libintl-perl/lib/Locale/TextDomain.pm>