Because $r is being used as an object, by calling its send_http_header() and print() methods. Without the shift (I presume this is inside a subroutine), the object represented by the reference $r would never be referenced and the code would not work.

P.S., without seeing the context in which your example lives, it I do not exactly know what kind of object $r is , but from its method names, I suspect it's one of the Http::XXX'en.

dmm

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