Keyword::Simple is super easy to use. You choose a keyword, say, "foo", and set up a handler for it like this:

package My::Keyword;
use Keyword::Simple;
sub import {
   Keyword::Simple::define(foo => \&_parse_foo);
}
sub _parse_foo {
   ...;
}
1;
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Then when someone uses your keyword (this is at compile time!) the _parse_foo sub will be called. It will be given a single parameter: a scalar ref - the reference to the string of Perl code currently being compiled. Now the _parse_foo can change that string, to change what gets compiled by Perl.

Syntax::Keyword::Val is a really simple example. The val keyword simply gets rewritten to Data::Lock::dlock my, so when people code this:

val $foo = 42;
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That Perl code is rewritten to:

Data::Lock::dlock my $foo = 42;
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Moops employs a lot of Keyword::Simple trickery.