... key points ... "map" [which] appears to create a hash ...

No "hash" (in the sense of an associative array) is created at any point. The critical effect of the map expression is to extract the format regex ($1) and descriptive text ($2) substrings from each data type specifier record and use them to build a sub-regex for each data type. The  (?{'$2'}) sub-sub-regex generates code that evaluates the descriptive text substring and returns it via the  $^R regex special variable (see perlvar). All these sub-regexes are then concatenated together into one big alternation.

You can just
    print $regex, "\n";
and pick your way through the result to see the alternation of all the sub-regexes in all their glory.