Dunno about Raku but I already talked here about this BS in PHP. (AFAIK they've corrected the documentation in the meantime.)

The term stems IIRC from Lisp was popularized in Scheme and means a "hull" structure containing a sub plus bound lexical variables.

Not sure if other languages also implement this with lex-pads like Perl does. (See e.g. PadWalker)

PS: I'm happy to see that these videos initiate discussions. :)

Update

see also WP:Closure (computer programming)