Keyword::Declare implements a new Perl keyword: keyword, which you can use to
specify other new keywords.

Normally, to define new keywords in Perl, you either have to write them in XS
(shiver!) or use a module like Keyword::Simple or Keyword::API. Using any of
these approaches requires you to grab all the source code after the keyword,
manually parse out the components of the keyword's syntax, construct the
replacement source code, and then substitute it for the original source code you
just parsed.

Using Keyword::Declare, you define a new keyword by specifying its name and a
parameter list corresponding to the syntactic components that must follow the
keyword. You then use those parameters to construct and return the replacement
source code. The module takes care of setting up the keyword, and of the
associated syntax parsing, and of inserting the replacement source code in the
correct place.

WWW: https://metacpan.org/release/Keyword-Declare