This implementation should work in any perl from 5.19.3 onwards. Earlier versions are likely to work too as long as you install a new List::Util. The implementation uses the experimental switch feature, which may make it vulnerable to future changes in perl.

Mal objects are all in subclasses of Mal::Type, and can be treated as scalar, array, or hash references as appropriate.

Metadata support uses Hash::Util::FieldHash to attach external metadata to objects. This means that in the metadata system imposes no overhead on the normal use of objects.

Hash-maps are slightly magical. They're keyed by the stringified versions of mal objects, and Mal::Scalar overloads stringification so that this works properly.

Tail-call optimisation uses Perl's built-in goto &NAME syntax for explicit tail calls. This allows functions defined by fn* to be implemented as functions at the Perl layer.

Perl's garbage-collection is based on reference counting. This means that reference loops will cause memory leaks, and in particular using def! to define a function will cause that function to have a reference to the environment it's defined in, making a small reference loop and hence a memory leak. This can be avoided by carefully undefining any function before it goes out of scope.