The module Lingua::tlhInganHol::yIghun is using Filter::Simple basically all that module does set up callback a function (FILTER) for a module ( let's say MyLingua ) and that function is called when a piece of code uses MyLingua. If you read the source code of Lingua::tlhInganHol::yIghun you'll see a big FILTER function with the contents of the parser. The parser transforms Klingon into Perl.