I like the elegance, consistency and similarity with natural language of Perl 6. E.g. the colon, with which we express a relation. Cups in the basket: 5.
And indeed a pair as a data type, which describes a simple relation, you declare in Perl 6 :key(value). Also hashes (which are build with pairs), named parameters in signatures (which too descibe a relation between a name(ID) and a value) autogenerated named parameter (have colon as twigil) use the colon. And not only the reuse of pairs everywhere as quote or regex adverbs or filetest operators, also in macro defenitions, if you specify what you like to create with this macro (because unlike LISP there is more than lists and brackets), you write macro category:
thanks jesse++ for adding me.