Second, guess, You are wrong, it's not built in function:

Well, yes, you’re right, it’s not only a built-in function, it’s any function, built-in or user-defined, that has a ($) prototype.

The Camel Book (4^th Edition, 2012) has a table listing “all the named unary operators” on page 106. After 88 built-ins, the final entry is:

any ($) sub

Hope that helps,