From type/Supply

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method watch-path($path --> Supply:D)

Creates a supply to which the OS will emit values to indicate changes on the filesystem for the given path. Also has a shortcut with the watch method on an IO object, like this:

IO::Notification.watch-path(".").act( { say "$^file changed" } );
".".IO.watch.act(                     { say "$^file changed" } );   # same